Chipmunk was one of the first I played with. I was initially impressed but something turned me off, I don't remember what (this would be over a year ago, so it's probably evolved since then). I think it was stacking. My expectations for stable stacking are probably too high. I briefly left some comments in their forum I think, so maybe I can go back and figure out what my issue was.
At another point I played with building a .Net wrapper around Box2D (Eric Catto's C++ library that Chipmunk is loosely based on, IIRC). Something turned me off this path, too. Might have been stacking again, but I can't remember. It also wouldn't have been XBox friendly (no unmanaged code allowed).
Then I spent about 8 months off and on (mostly on) playing with the idea of a non step based physics engine. Basically I'd calculate exact times of impact for all collisions projected indefinitely in to the future. It works pretty well for circles moving without drag (involves solving some quartics), but anything more complicated is difficult to impossible with present technology (a great deal of original research would need to be done. It's probably a masters or PhD thesis amount of work.)
So now I'm back full circle to thinking about a premade library. It needs to be all done in .NET so that I can load it on to my XBox, which leads back to Chipmunk probably.
BTW, it's nice to have someone else who's physics knowledgeable. When I started with Darwinbots I knew absolutely nothing about physics (my first big breakthrough was realizing you didn't need a square root to compare distances between points with other distances between points. And using vectors was big, too.) That was maybe 4 years ago or more, and I've learned a lot, but I'm still just playing it by ear.
ah, I figured it out. Chipmunk isn't native .NET, so you need either a port or a wrapper for it to work. I got part way through a wrapper, then got frustrated and went to build my own. Then got frustrated and went to Box2D for some reason and built another partway wrapper. Then got frustrated and went to build my own again, then got frustrated and here we are.