One thing to watch out for is the concept called (IIRC) "universal default". Under this philosophy, any default, late payment, etc, on any of your obligations triggers a rate increase on your 0% card.

So, you've got a substantial amount out there at 0%, but for some reason your phone bill slips through the cracks one month. Fifty bucks. No big deal. Suddenly your 0% balance is now at 13.4% or 21.9% or something like that.

These guys just love to do that.