My take on the karts...
Overall AMAZING, SUBLIME, A PROPER SIM-CAR REGARDLESS OF WHAT SIM GAME / ENGINE YOU ARE A FAN OF. IF ANY ISI-PHYSICS HATERS (don't understand how anyone could even dislike RF2 physics, overall) DON'T LIKE THIS CAR AND THINK IT'S CRAP THEN I'M SORRY BUT YOU ARE JUST BIASED AND A HATER/BASHER/FANBOY BECAUSE THE PHYSICS OF THIS ARE AMAZING REGARDLESS OF WHAT SIM YOU NORMALLY LIKE / DISLIKE. EITHER THAT OR YOU JUST SIMPLY HAVE NO IDEA WHAT A REAL KART IS LIKE.
Most people can see and feel how amazing and true to life they are, with the steering angles, the way the grip comes in and out during different steering inputs, during different throttle and brake balances (especially during the initial turn-in and the second part of turn-in as you are making your way towards the apex). I am not going to go on about details on how they are so amazing because most people can already see and feel it. However I am going to voice my personal opinion on just three things that negatively stick out for me....(I think the issue with point #1 is more an issue with the underlying RF engine, rather than a kart specific problem, but I believe the Karts being so "hard-edged" tend to bring this issue out more than most cars).
1. When you get in a "bigger" oversteer moment. You sometimes feel like you need to apply, let's say 30 or so degrees of steering lock to save a big slide (not a tiny twitch slide), but then as soon as you get to about 20 degrees as you are applying the lock them BAM all of a sudden it regains grip and it just instantly snaps you the other way in a mega lightning fast overcorrection. I have noticed this physics phenomenon in many RFactor 1 cars, where for some reason the lock needed to correct a big slide is less than it actually feels/seems like is needed. FOr example it feels like you need 45 degrees of steering angle to correct a slide in whatever car, and as you are correcting and turning your wheel towards 45 degrees, all of a sudden when you are at around the 30 degree mark (for arguments sake) the car will out of nowhere just almost instantly and out of nowhere regain it's grip and just correct and just snap the other way. If it doesn't snap you into an over correction spin then it will just send you into this weird oscillating, real quick, left-right-left correction rather than just the normal "save slide with initial steering lock, and then correct back to centre, done" technique.
This would happen many times in RFactor 1, even the most amazing mods/RFactor 1 based products would exhibit this behavior once you got to a certain point (although some definitely did a better job at hiding it, almost to the point most would probably never experience/notice it). RFactor 2 seems to have massively fixed this problem from RF1, however it seems that the harder edged the cars become the more this phenomenon seems to rear it's ugly head. You can feel and just tell as you are correcting a slide how much lock you need to correct it, but as you are heading towards that lock the car then just re-grips seemingly early out of nowhere and therefore corrects so harshly that it can lead you to massive over-correction (sometimes causing a massively quick over-correction spin, other times, as you get used to it, just causing a very un-natural save of the slide where you apply such little lock and it feels very overly touchy and artificial). You can see this easily when watching open wheel car slides and corrections on TV. Even when we have the correct steering lock in-game which matches the real-life steering lock and most of their regular steering inputs, when it comes to saving a bigger slide we always seem to need such a smaller amount of steering lock to correct that slide. Netkar Pro and the super early version of Driver's Republic (same physics genius that did Richard Burns Rally) pretty much nailed this whole "steering lock angle to correcting a large oversteer moment ratio" (notice I said Netkar Pro, NOT Kunos's other sim.)
The karts seem to bring this physics phenomenon out. Once you get used to it you can prevent the karts, for the most part, to getting to that point and then you will not notice it and everything is fine, but it really limits you in just how far you can go with the oversteer as going too far with the oversteer will just snap you to the other end with the weird correction physics phenomenon buried deep in RF's physics engine (going back before even RF1, but thankfully always improving).
Once I took the auto "vehicle set" steering lock off and just set it to 300 (I think vehicle set was setting it to 187) then it did feel a bit better, but now the steering ratio quickness was compromised in terms of realism in order to try and adjust it to help with a seeming physics flaw.
Having said that, I raced from about 8 am to 4 pm in the karts yesterday and had an AMAZING time. Once you get used to the kart and keeping it within it's limits (you are still constantly going over the limits and correcting perfectly fine, remember I am just being picky here) and away from that area of oversteer that I mentioned above then they are so unbelievably good! For those 6 hours of online play I probably only got in that situation less than 10 times after the first hour of getting used to it. So honestly it's not a huge deal, but it's this underlying physics phenomenon that has gone back from even before RFactor 1. It's buried deep, DEEP, in the ISI physics engine.
2. Sometimes when you are on the grass at literally like 3 Km/h you just go into this mega lighting spin. I am literally pressing my throttle like 5%, my toes are barely touching my pedal and my speed is literally under 5 km/h and BAM lighting quick spin out. I don't care who you are there is just no chance of correcting it. Other than this and what I mentioned above it's just heaven - 9.5/10.
3. I do feel in real-life that you can have the kart come into the corner in more of an angled drift, and steadily hold it there, rather than having to do snap corrections and always being "scared" that you are going to loose it and it's going to snap the other way. The tyres do allow you to maintain that sideways momentum longer in real-life, where in RF2 it seems like it's more scary and that the car might just snap on you, either into the corner or the other way upon the weird snap over-correction. This issue might have a direct connection to issue #1 posted above. Having said that, you can still use the rear to go sideways into the corner like real karts, it's just things constantly seem more snappy. (Maybe part of this is just down to lack of G-Force and relatively laggy wheels and PC's relative to real-life, so I cannot blame the physics 100%).
I cannot fault most of the grip situations in any way, not the way it regains grip, the way it looses grip. the entire chassis dynamics from initial braking until exit, and so on and so on, nothing I can critizize at all, (except again if you ever get into the situation I explained above, more of a general RF physics issue than a specific Kart issue though). Almost everything is absolutely sublime. Just the tinniest bit of understeer on the exit pushing me wide off the track and the quickest lift of the throttle and back on (literally as fast as I can do it) and I am back on line, the rear rotation into corners, just everything. They are not only the best karts of any sim ever, but some of the best most realistic sim cars of any sim ever. I still have some issues with FFB feel, but in terms of vehicle behavior modelling (yes, unlike some people, I know how to distinguish pure in-game vehicle behavior from my own personal FFB "feel", ahem you know who you are) they are some of the best sim cars of any sim EVER

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Regarding the track graphics. ISI already stated it's a very, very old track so I wasn't expecting much. I was playing on mostly everything medium with both refelctions off and absolutely no Antialiasing whatsoever. Well I am not going to say much else because ISI already know it really needs to be improved, but what I am going to say is that after I rolled out of the pits on my very first ever lap I never noticed, or cared for, the grpahics again. I was concentrating on driving so hard, where to find tenths, where I am loosing out, how to slightly change my technique for a certain corner in order for the kart to do a certain thing, etc. etc. that the graphics never, EVER, even crossed my mind. The driving was amazing and nice lighting, textures, refelctions, etc. would not change any of that. However a few hours later when I took a break and went into manually TV cam mode (controlled by mouse), then yes sadly I was reminded about the graphics, but that was the only time. One thing to point out, that blue and red balloon thing you go under looks VERY realistic in how it moves around from the air and wind and such, especially at 120 fps / 120 hz. Extremely nice touch to detail regarding the "movement graphics" of that balloon arch thingy.
Final Note: Forgot to mention, in my experience of a real rotax 125cc the powerband seemed to kick in much harder and aggressively once you got into the power band (not sure exactly what RPM as I wasn't looking at the dash, actually I could hardly even read the dash as everything was vibrating, moving and bouncing too much, it really is that rough in real-life. Probably around 7 - 10 thousand RPM), where as these feel more constant and smooth as it rises from minimum to max revs. Then again I thought the same regarding the Game Stock Car karts, so maybe they infact are modelled properly and it's just that I can't feel it properly due to lack of g-force, vibrations, sound modelling, etc.