Just a few days ago I used to use a controller. In fact, I've been doing so for the past year or so. It's definitely possible. The issue is that you'll never be precise enough with it. Trying to go incredibly fast and incredibly smooth through a corner takes guts with a controller. The limit before you lose grip is much lower than if you had a wheel. I just got my wheel a few days ago and, after dialing back the FFB settings, already I can see an improvement. I'm probably still not as fast as I was with a controller, but I once I get used to it (and once I get a gear shifter for all the cars that require it), it'll be very easy to outperform a controller. The general rule of thumb is that even the fastest controller times is about on par with a sketchy or average (not particularly quick) wheel time. So to answer the original question, you probably won't be competitive with it, but it is possible to play the game with it. But it takes a lot of practice to get it down. Some games, like Automobilista or Assetto Corsa are actually pretty decent with a controller while games like rFactor 2 are not.