Always have to go manual. I find in this game you get far more control over the car, being able to over rev once in a while when you need it can make a difference, and being able to pick your gear for a corner makes a huge difference. Eg, you pick high revs in 2nd for a corner (maybe real close to the rev limiter the whole way through, but engine can't quite rev out) where computer takes 3rd and bogs down. Also helps you power oversteer the car around corners which helps with high power FR cars.
I think if you spend a lot more time driving you'll get to the stage where you can't do the goods unless you're using manual. Suprised to see the votes are almost even. Mind you I started driving using the joystick for gas/brake control before I got the wheel, because I wanted more control over the car. Took me a long while to learn but I felt its worth it. I'm faster that way than using the buttons for brake/gas.