I have been driving 500pp London quite a bit, but I almost never win. Most of the drivers are in the boxy Mitsubishi thing, and I'm simply unwilling / completely uninterested in, driving that. It simply destroys anything I've tried on every straight. Also, most of the driver just drive over the curbing, something which would have caused major problems in GT4 but which seems way too forgiving in GT5p. So I've been happily out there losing (usually in my Lotus).
I've also been really enjoying Eiger, which I always wanted to race online. Yeah, it is slow cars and standard physics, but get even 4 racers out there and it is a blast. It has also turned into a "1 car" race, in this case the Mini, which the other cars just can't seem to touch.
750pp HSR reverse is where I can frequently bring in the gold, or at least a trophy, so I try to make it there for at least a couple races each evening.