Seems like a pretty long game :) How do you feel about more 'standard' racers like those with a rally or street-racing theme? I suppose the best racer I've ever played was 'Most Wanted' or 'NFS underground 2'. Or perhaps Star Wars ep I racer come to think of it.
GPL can be as long as you want it to be (you can even make a season that contains all 500+ tracks released so far, even though it may take years to complete it; not exactly "a season") and as with every sim (racing or flight), the replayability is endless.
About the more "standard" racing games (arcade and such), i have mixed feelings about them; there are some arcade-y games that i actually enjoyed, but i don't usually spend much time with arcade racers in general because i don't find them as engaging as racing simulations. Maybe it has something to do with the fact that i enjoy riding motorcycles (on road & in track) and generally driving as a hobby rather than only as a means for commuting, and this reflects to what i expect to see in a racing game, especially after all the experience i've gained in real life.
Anyway, if you're curious about some arcadey games that i've enjoyed, here's a list (i played quite a lot of racing games in PSX back then, but i forget the titlenames of almost all of them):
☆ Gran Turismo 1, 2, 3
☆ Colin McRae Rally 1, 2
☆ 4x4 Evolution
☆ Wipeout 2097 & Pulse
Regarding the NFS series, i used to play the older ones (such as Hot Pursuit) back in the day, but i find the newer ones uninteresting. I recently tried Most Wanted when i visited my parents (my father is an avid gamer) and i didn't like it at all. And as for the Star Wars Racer, i remember myself having lots of fun with the Dreamcast version (along with a game called Hydrothunder, if i recall the name correctly) and being speechless of its graphics.
Having said that, at the end of the day i'm still returning to GPL, NR2003, P&G3, RBR and GP Bikes. (Too many acronyms, i know. :P)