I haven't put any other game in my PS3 since I bought GT5, but I can't honestly say I haven't WANTED to...!
If it weren't for the arcade-y handling, and how that would mess up my carefully practiced GT5 chops, I might have put Shift back in and played that a bit online. I'm sorry to say, but I found online racing in Shift FAR more addictive and just plain FUN than I have found GT5 to be. I know, a fair bit of it is simply getting used to the more realistic handling (but I raced on Shift with all aids off other than Low steering assist, to get rid of the DFGT 'wobbles' that didn't plague the pad users), but a LOT of it is about just how easy Shift is to get a race online, driving pretty equal cars against each other, and the online leaderboards, where, if you set a personal best, you'll climb the ladder. I have several Top 50 times, and it's always fun to try and get just a BIT faster.
Manufacturer Races, where you drive the stock version, was some of the tightest and most fun racing I've had, and I've yet to come across anything online with GT5 that comes as close. Sometimes, TOO much choice works against you. 1000 cars, and extensive personalization of upgrades can make for a pretty difficult time finding a good match.
I'm sorry, but I'm finding it harder and harder to resist putting Shift back in (and pre-ordering the new Shift, too). I find I play GT5 maybe only about 10% of what I used to play Shift... Driving physics isn't EVERYTHING, and it sure doesn't equate to FUN if the rest of the game is flawed.