As far a games go sim racing is about the only thing I get into. I don't have Heat though. I do Papyrus Nascar 4, and I only race online, hardly ever against the computer.
And I agree with Janus GPL although being 4 or 5 years old is still one of the best racing sims on the market. Nascar 4 uses about the same car physics as GPL and is the best Nascar racing sim to date in my book.
Running Daytona or Talladega at 200 MPH in the draft with 40 other online players waiting for the big one to happen is white nuckle heaven or hell if you happen to get caught up in the big one. Wins are as much luck as they are skill of driving and creating a good setup considering the plague of online professional wreckers who's only purpose are to take out as many of the leaders as they can.