So I bought a GTX670 just for this game

It's the ASUS DirectCU non-TOP version and I absolutely love the card!! My previous Toxic HD5850 was no slouch but this thing is just awesome
Borderlands 2 runs all maxed (PhysX high) at 1080p usually 50-60FPS. I cap it at 60FPS, no point in having the card produce more anyway. However there are some places where the FPS drops to under 30 (I've seen as low as 25...), especially when there's a nice vista (like the very first "city" you find). My CPU runs at 3.2GHz but I guess the Core2 architecture on my Q9450 is showing its age here? When the game slows down this much, GPU usage usually doesn't go above 50%... I've seen it as low as 35% D:
However, 99% of time the game runs brilliantly - I really really like the debris and particles and all the other cool PhysX effects. Sure, they are eye candy only, but so are all the other settings too... AA, texture, LOD, etc.
I don't regret getting the GTX670 one bit

It allowed me to enjoy Borderlands 2 more than I would with the Radeon. Plus all the other games run better too ofc (what a difference in Skyrim for example!).