Ok, I haven't read any of the FB other members have posted, but here are my 2:
I had been a religious GT fan, until I saw FM2 in action. Kinda went back to GT4/5 for a while, until FM3 made me drop my jaw all the way to the floor. I did however, continued to play GT5 now and then.
Then came FM4, and oh my Lord... GT5 has been collecting dust ever since!
The first thing that's going to immediately blow your mind is the tire model, suspension physics, and g-forces you feel coming into and going out of corners. I know it's not an actual simulator, but once you get lost in it, it IS the closest thing you're going to experience shy of actually driving a really fast car.
Then, the engine sounds... get ready for this, because FM4 delivers engine sounds like you've never experienced before.
The wireless wheel is OK. Get a fanatec CSR Elite intead, along with the pedals. Make sure you get the latest update, as that allows you to use 900 degs of rotation with steering AND counter steering.
Cheers mate, hope this assists you in making up your mind.
GET FM4 today, you will NOT regret it.