I'm about 27% complete so far, with about 150 cars, all but two of which are prize cars or used cars. Only a few pure race cars, though; most of my rides are stock or slightly tuned.
My favorite cars I've driven so far:
Subaru Impreza Super Touring Car - This thing is very quick, even without any upgrades, and handling is not bad at all once you tweak it a bit to get rid of the oversteer (thank God for VCD...).
Buick Special - This car is a blast to drive! Insanely fast and handles terribly, but that just makes it more fun. Kind of reminds me of the Escudo in GT3, only the Buick is actually driveable once you get the hang of it.
Ginetta G4 - Won this as a prize car and ran it in the Euro lightweight cup. Added some HP mods for the hell of it, and damn, this thing is cool! It's incredibly light and twitchy, and very fast for its size. Had a blast sliding around Autumn Ring with it.
Chevrolet Silverado Concept - It's not just "good for a truck", it's actually a pretty good racer in its own right. Suprisingly stable and nimble for a pickup. Just don't let the revs drop below 4K in first or you're gonna need a push-start to get going again...
Mazda Mazda6 Touring Car '02 - Not the most powerful car I've driven, but holy crap, this thing drives like a slot car. I don't think it can be wrecked. NOTHING I do will get this car even a little bit unbalanced. Diving into the sharp left-hander at the end of the steep downhill on Costa Di Almafi I with the brakes fully locked didn't even make it twitch, and doing that in any other car is almost certain to leave your ass end facing the wrong direction. I even took it to Tsukuba Wet, and by applying the handbrake at full throttle in the last sweeping turn, I was able to get it to spin a whopping 90 degrees before it came to a stop. And this is all on the Racing Super-Hard tires it came equipped with.
Mazda RX-8 Concept LM - Just won this car in the RX-8 cup, it's a bit faster than my modded Subie touring car and handles better out of the box (hooray for oversteer!). Can't wait to run it in some real races.
And some disappointments:
Plymouth Super Bird '70 - I've only driven a few of the good old American muscle cars so far, but this one is by far the worst of 'em. Handles terribly, and is really pretty slow to boot.
Plymouth Prowler - WTF? It's a production car, but I can't race it. That sucks.
Nike One - While it's interesting as a future car concept, why the HELL is it in a Gran Turismo game? Concept cars I can understand, but a car from the future that's built using technology that doesn't even exist yet? Gah. Leave that to the arcade racers, please. The Toyota MTRC is almost as useless in the game, but at least it actually exists in prototype form in the real world.
Cadillac Cien - A powerful and good-handling supercar that would be a superb vehicle if it weren't for the fact that I could read a typical Stephen King novel in the time it takes to go from second gear to third. This thing loses 5+MPH while shifting on a flat road, and heaven help you if you need to change gears on an uphill. What's the story with this thing, anyhow? Did they put so much money into that ass-ugly styling (which sadly became the new Caddy "look") that they had to scavenge a transmission from a Dodge Aries at the local junkyard?
Land Rover Range Stormer Concept - Yuck. Drives like what it is, a huge, heavy, poor-handling SUV. Not to mention the fact that it's sprung softer than a Volvo 240 wagon. Don't drive with the bumper cam if you get motion sickness!

300 horses and it can't keep up with a Toyota Celica with half the horsepower.
Option Stream 350Z - 770 base horsepower! This thing can really burn up the road when the turbo kicks in. Too bad that doesn't happen until 500RPM below redline. I think my $8700 RX-7 starter car can outrun this thing 0-60. What a waste of a car.