When I was younger, I used auto. Then, I decided to start using Manual with GT1. The last game I used manual in was Rallisport. However, I had a lengthy discussion with a friend. We eventually came to the conclusion that automatic and manual transmissions don't act the same in the real world and video games.
For instance, in the real world, auto transmissions are typically bogged down by torque converters, less gears that are wider and less intelligent switching systems that are a bit laggy. However, in the game, the car revs to redline before chaning gears. There appears to be no loss of power due to torque converters, nor do the transmissions feel sluggish. Essentially, the computer knows where you ought to be transmission wise, and will make appropriate decisions.
Rallisport was quite fun switching around, but GT1 was not. I've found that only a few games really are fun with manual, and most are just fine with auto.