I suggest you put all aids on, except stability control, since it slows you down (but it could help you). When I got my wheel in december, I removed all assists, like I do on the controller, but I kept crashing and I was like: "How can I keep the car in a straight line, heck, I can't even take a corner without spinning and ending in the barrier"... Then I stopped using the wheel and kept going at the DS3 (control pad). Then I had an idea... "why shouldn't I use assists ? who cares ? Do I care ? No. And what will other people say ? Who cares ? The point is to have fun and enjoy the game as well as the wheel you bought. I sure enjoy my G27 now, I've had more fun in the last 2 weeks (got my wheel in dec 2011) with assists on with the G27 than I had with a DS3 from launch day in nov 2010 up to december 2011.
Well, guess what ? With assists on, I have SO much fun, that I think I will never (ok maybe sometimes...) go back to the control pad. That way, you'll have fun and develop your skills. When you feel confident enough, remove the aids you don't want anymore.
G27, stock cars and assists on, GT5 has never been better.
PS. As for drifting, if I'm not mistaken, top players uses the DS3 to do so and I have NFS S2 and drifting is HARD, so don't worry if you cannot do it with a wheel. One thing I miss with the wheel, is that I cannot use the E-brake as much as I did with the DS3...