If you haven't got them already, I would recommend going out and buying some attachable triggers for the L2 and R2 buttons, they bend in the other direction like the 360's two trigger buttons but are much chunkier. They really help to modulate the accelerator and brakes better, although GT5's sensitivity is way out of whack in my opinion.
The brakes work perfectly as the L2 button, the amount of pressure you apply translates exactly and equally to the brake peddle - in other words, if you hold down the button by 25%, you brake by 25%, if you hold down the button at 50%, you brake by 50% and so. With the Accelerator, I find the sensitivity jumps up and down. There's too much dead zone in the first 25%, 50% isn't quite 50% acceleration, and then the sensitivity jumps up sharply after 50% pressure on the R2 button. If you are half throttling perfectly and put on the tiniest amount of pressure, the on screen graph on the accelerator jumps up way too far for the amount you're pressing, and then flickers up and down if you try and correct it. At least that's what I find anyway.
Quite often, it's one of the only things that hinder my performance, sometimes when I need to hold half throttle through a long corner, I find I'm either accelerating just not quite enough and lose a little time, then start applying slightly too much acceleration trying to hit the perfect amount of pressure on the button. If the pressure worked the same as the brake peddle, I would have no problems at all.
Edit: Seems the person above me has exactly the same problem. It is GT5's interface and response, not the controller itself.