For me anything over about 45 FPS is good enough, 60+ is lovely. I realise it's a personal thing.
With Quadviews, I'm normally over 70FPS.
I've managed to switch off reprojection or whatever it is that locks you to 45/60/90/120 FPS and I find it much smoother with the FPS being able to range. With reprojection I always found a noticeable jitter when looking left or right and seeing scenes with lots of moving objects ie the terrain moving accross your vision.