For me the most disappointing cars in the game are the Caterham Fireblade and the Tesla Roadster, as neither is clearly set up how it should be, and underperforms.
On the flipside, the '74 Countach and the Miura are better than they should be, as PD got the weights wrong (way too light). This seems more noticeable in the Countach though for some reason.
As for the Veyron, i'm still totally mystified by people bashing it. For crying out loud, it seems to understeer because you are approaching those bends MUCH FASTER than you would in any other street-legal car. Sure it's not a light car but there are far worse handling cars (give it to me over an Enzo, in GT5 at least, any day)
I tested 100 cars recently at Trial Mountain (all stock, used cars had rebuilds where required, SS tyres the only mod and all aids off except ABS at 1), comparing hot laps and the Veyron was the quickest on what can hardly be called a very straight, open track. There were a handful of cars within a couple of seconds, then a little gap, then the rest of the pack. The McLaren F1 was, surprisingly, a little down the list. I suggest some of you bashers do a fair comparison also, with clean laps. Just remember to brake a little more because you will be going so damn fast
EDIT: And if you do compare, make sure it's with PRODUCTION CARS! The Veyron is a street-legal car, not a racing car. Maybe then we'll see less posts along the lines of "I couldn't win Like the Wind in my Veyron blah blah blah it's meant to be the greatest fastest car ever, so what's up? Duuuh..."
Ahem sorry.