Obviously they have used real cars as templates for their designs, but their choices never cease to amaze me. Rockstar North must employ an entire team of Jalops. For example:
The Toyota AE86 clone, the Karin Futo. It's ridiculously fun to drive, and very easy to drift.
There are also some crazy mods, like Shakotan style exhausts and bumper mounted radiators, and "Dakar" style mods for the Toyota Hilux clone, the Karin Rebel.
While the the driving physics are less realistic than they were in GTA IV, it's not terrible. Left foot, er, finger braking is a good way to get the car round corners.
While you can tell what each car was based on, some are more realistic than others. Many of the supercars have a split design, as in the front will be based on one car with the rear coming from another. For example, the Peggasi Vacca. The front is a Gallardo, everything behind the doors is a McLaren 12C.
Here are some of my favorite cars that are very similar to their real life counterparts.
Lampadati Felon - Jag XF. It looks like they didn't even try to hide it either.
Bravado Gresley - Current Gen Dodge Durango. You can Argue for the Bison/Ram as well, but all pickups look the same anyway so that doesn't count.
Imponte Ruiner - Pontiac Firebird F Body
Benefactor Shwartzer - E Class Coupe (Most websites say CL or C Class coupe, but look at those tail lights - definitely E class.)
Obey Tailgater - Audi A6
Vapid Bullet - Ford GT. Been around $ince $an Andreas, still awe$ome.
Enus Super Diamond - Rolls Royce Ghost or Phantom, I don't know the difference because I'll have to worry about buying one...
Bravado Gauntlet - While the tail lights look like they were lifted off a Camaro, this is clearly the Dodge Challenger.
Vapid Dominator - Mustang, duh. There's even a mod for grille mounted fogs.