I've been with Dreamhost for over two years now and have been very happy with them. However, if you expect instant replies from support (or if you need a lot of very basic hand-holding), Dreamhost may not be the way to go. I've found their service very reliable overall and their support has always been there for me. However, I know what I'm doing -- well, most of the time.
I've experimented with a number of CMS systems on my Dreamhost accounts and they have all worked well -- at least provided that they are php-based and fairly well-written. One perl based one timed out on updates a lot, but I've been told that that system was poorly coded. Mambo worked fine, but was far more than I needed.

One thing to watch out for is limitations on MySQL usage hidden in the fine print somewhere. I've seen some hosts that offer huge amounts of disk space, but limit your MySQL databases to some tiny amount of that. That can really cramp a CMS.