I had the exact same experience as the OP. Nice call, got rates droped, next bill was higher than rates before they agreed to lower them. I called. Refused to speak to anyone but a manager and then just lost it. I ended up with the lower rate I was promised and more premium content. I'll be in the same soup again soon though, as it was a one year deal. My bill will skyrocket if I don't head it off at the pass.
I don't mind negotiating in general, but the time it takes to deal with Comcast and their system is truly (and I'm sure intentionally) frustrating. I'd like to see fixed rates and no promos or specials. Won't ever happen. Maybe competition will though...some day.