I completed about 65% of GT3 using Auto transmission before school and other commitments got in the way of my gaming for a few months.
When I came back to the game, I made the conscious decision to teach myself Manual transmission. I spent about a week slamming into walls, spinning out, losing races by +X laps and generally making an ass of myself before I finally started to improve. Now, several months on, and having started on GT4, I can't imagine racing without MT. It suits my driving style.
And that is essentially what this comes down to; I recommend that if you're not sure about manual, try a few practice sessions with it in a slow car until you get a feel for when to shift (for me, the upshifts weren't a problem, but the downshifts proved difficult). When you feel yourself improving with it, THEN you can ask 'is this right for my driving style? Will it improve my times?' If not, then go back to automatic, safe in the knowledge that you will be racing faster on AT than on MT.
And that is essentially what this comes down to; I recommend that if you're not sure about manual, try a few practice sessions with it in a slow car until you get a feel for when to shift (for me, the upshifts weren't a problem, but the downshifts proved difficult). When you feel yourself improving with it, THEN you can ask 'is this right for my driving style? Will it improve my times?' If not, then go back to automatic, safe in the knowledge that you will be racing faster on AT than on MT.