I also found that the Firefox Convergence add-on (which itself is a neat concept) breaks client certificates (which I keep forgetting, so when I hit the private areas of my secure server, I keep getting denied until I remember to disable Convergence).
My experience as well. I tried to implement client side certificates for an internal system (changing computers/browsers/ugly UI/etc not an issue), but couldn't quite get it to work.
And in that setting not worth the time compared to Basic Auth.
