I too have switched over to Forza 4 from GT5. Having played every Gran Turismo game in the series, and loving every one of them, GT5 was in my PS3 for about 7 months straight, before I even thought about playing a different game. But after completing most of the A-Spec races, and doing some seasonals and whatnot, it just became kinda of boring. Granted, the graphics were amazing on the cars that were premium, but graphics don't make a game for me.
I then played Forza 3 on my brother's xbox 360 awhile back, and although the visuals weren't quite as stunning, it still managed to hook me. Then I heard about Forza 4 coming out, and around Christmas time last year, I found a great deal on a 250gb Xbox slim, and Forza 4. Now, just as it was with GT5 and my PS3, Forza 4 has not left my xbox all year so far.
Overall, I'd say the Forza series is just way more fun, to put it simply. Every car has the same level of detail, the physics are awesome, the selection of car keeps growing by the month, the amount of customization and tuning is better than GT5, etc. I will say that I wish there were a couple track packs coming out or something, but I still love the game. Oh and the livery editor, that's a major plus as well.
All in all, it was well worth the purchase, I have not regretted it one bit, and I have definitely played my money's worth. You will have a blast!