So here's my deal. I stopped playing racing games back in the GT2/F1 2000 as I just didn't have the urge anymore to play racing games because it's simply dull. Being very young at that time, I didn't know if there is a wheel for ps1. So basically, I stopped playing racing games, ignoring and simply cutting all connections about racing games until I decided to give it another shot and invested on Logitech G27 early this year then bought GT5. When I got it in the mail, I simply didn't have any feeling about it, I still felt that it will not change my view on racing games.
So I opened the box, was a bit impressed on how the set felt 'kinda' real because of its weight. Real quick about my background on driving/racing toys throughout my childhood.
Started with that toy with a shifter+wheel that you just put batteries, turn the key, and the loop of a track appears on the screen (you know what i'm talking about? yes, its old). Followed by matchbox, riding toy cars with pedal, then riding powered toy cars, RC cars, tamiyas with tracks and all, then onto video games, does sitting on my father's lap when I was around 7 on a real car while it is moving and controlling the wheel counts?
So giving it another try is a big deal to me. Opened the box, installed GT5, then the time came. Wow, did I just waste a decade by not keeping in touch on racing games?? Being a very seasoned stick shift driver in real life, I had no problem using this thing. It reminded me how I use to love driving games. So when I see my friends playing with Forza or GT5 without a wheel, I always remind them to invest on a wheel.
So to answer your question, you really have to try both for yourself. I used to love racing games with just the controller, but having a wheel will literally make you wonder, why would you play a driving game using a controller?