Most definitely, a buy-and-keep it sort of simulation.
Sound detail is ambitious and mostly spot on. Those of us who have driven a BMW CSL or a Ford Escort RS (even for a few laps only) recognize the sounds immediately. The same thing happen with other cars modeled, as people have stated.
But not only the sound. It's the feel too. It's not having all the modern technological marvels that help the driver/pilot. It's close to what people experienced back in the 60s and 70s.
Someone (a user) reviewed this and commented about idiots with computers. Well, some of those the user called "idiots" really have real life experience with fast cars and some of us have real life experience with GT cars. And we say this: although not perfect, the isiMotor2 game engine and SIMBIN simulations are quite close to reality - if not spot on in many areas.
Try iRacing and drive a legends car and compare it to similar cars offered by GTL: the similarities are striking (snap oversteer, understeer, all modeled quite well in both titles).
Graphics wise, you won't be disappointed. Even when compared to GRID, Forza, iRacing, GTL (much as GTR2 and, to a degree, rFactor) , GTL ranks ok. Cockpits, body-works, tracks, landscapes - all beautiful with a touch of photo-realism.
AI is competent (human-like sometimes, and far from murderous as we experience in LFS).
Physics, as I have hinted at, is very good. We know of the limitations of the non-theoretical tire models (David Kaemer, from IRacing, is even now working on a real theoretical model, which he hopes will bring "EVEN MORE realism" to his empirical model), but Pacejka models (combined or non-combined) do the job pretty well. They are not perfect, they do not rate 20/20 with ALL situations, but are good enough to train pilots. So, good enough for the hardcore simmers.
The variety you get with GTL, the graphics, the environment simulated - all this points to one result only: BUY IT. And keep it.