Better than expected:
Camaro SS '10. I figured this would handle about the same as the '08 Challenger; fun in that muscle car way, but sluggish and not very fast. As it turns out, however, it's a different beast entirely. Swift as hell around corners, very nice and balanced FR feel reminiscent of RX-7 Spirit R or Shelby Series 1, and doesn't feel like its weight (~1800 kg) at all. There's something magical about the 3rd gear of this car: it seems to be the right gear for so many corners and it's never hard to decide what gear to use in any given corner. One of the most straight up fun cars to drive.
McLaren F1. I expected it to be fast of course, just not that it would smoke the street legal competition entirely (save for the Veyron; crappy handling, but 1000 horsepowers speak for themselves) given it's a 16 year old car at the time of the game's release. Swift as hell handling, total race car performance brakes, perfect over/understeer balance, fun as hell to drive, it's practically perfect in every way. On most tracks of average length it's usually some 3-4 seconds ahead of even the fastest road cars. Hard sports tires may be a little underperforming for the insane speed it's capable of (wouldn't surprise me if it came with grippier tires IRL), but I've learned to work around that. A top 10 favorite car, for sure.
Worse than expected:
Enzo Ferrari. I did some 40 laps on Tokyo R246 and still couldn't turn this car properly to save my life. This car turns in one of two ways: 1. understeer like hell or 2. oversteer and send the rear end flying like crazy if I accelerate just a hairpin too much when taking a corner. I get they were probably going for an F1-esque feel, and that it's not supposed to be "tail-happy" in its handling. However, in the game Enzo just understeers way too much for that to work. The 458 Italia I have a similar problem with, although not nearly to the same extent, and it's still a very good car. As for a truly awesome Ferrari: F430 Scuderia. One of my favorite cars in the game and handles nothing like the Enzo.
All of the newer Lamborghinis. I can get some pretty decent lap times if I use the right driving technique, but I never get a good feel for them. They have the worst kind of 4WD understeer possible, seemingly without any of the benefits of a rear mounted engine. Sluggish is the key word here. I've since learned that Lambos being a sucky ride isn't news to many people, but this is the first time I drive one in a decently realistic game. I do love both both Countaches, though (haven't tried out Miura properly yet). Also, Audi R8 is a way better alternative for 4WD, rear mounted engine, actually one of my favorite cars in the game.
(all cars driven stock, sports hard tires, aids off except ABS at 1)