The SLR McLaren has 2 things in its favor: braking and grip. Its lateral grip is phenomenal for a car approaching 1,600 kg, and its brakes are great for hauling this dreadnaught down from speed.
The W12 has fantastic characteristics. I don't really have anything bad to say about it, except for the fact that it cannot be used for Supercar Festival, even though you have to face it.
The Cizeta V16T has magnificent acceleration, but some of the worst brakes in the supercar class. Its handling is also very wooden, at least according to my limited experience with this particular car.
The Ford GT does indeed have the tendency to snap into a flat spin at the slightest sign of elevation change, but as has been said: when you learn how to drive it correctly, it is nothing but fast. Brakes are good, handles are tight and acceleration is at or near the top. With a top speed in excess of 200 mph it is a force to be reckoned with.
The Saleen S7 is (IMHO) a dummied down GT with slightly reduced acceleration and increased understeer. Nothing terribly remarkable, and nothing terribly devastating. Simply mundane.
The Cadillac Cien is also an unbelievable beast, but it is nearly undriveable on anything short of R-grade rubber. Understeer is prevalent and shift lag is beastly. Top speed is incredible. Still, it is ineligible for Supercar Festival, but that does not stop the computer from junk-punching you with it.
The M5 is the unsung hero of the SF-eligible entries, capable of snatching 200pt victories with relative ease. It's heavy and lacks in top-end when compared to other monsters in the class, but it is still a wonderful machine with stable, predictable handling and respectable acceleration. Great choice for anyone with the requisite skills.
Just to name a few...