I've never used them -- but my friends constantly complain of disconnects, slow speeds, shotty tech support, high rates, etc. While I'm sure these types of problems are often localized, I've heard the same complaints from geographically diverse friends.

My only problem is that they're a monopoly slowly reconstituting itself like the T-1000 from Terminator[1].

1. http://en.wikipedia.org/wiki/T-1000