www.siteground.com hosts problably 30,000 Joomla/Mambo sites. I have three domains with them. They are a true host in the sense of server admin (their noc) and service.
Overall their info architecture for user admin is pretty tight, (db, file management, mail, stats (awstats) fantastico preloaded). cPanel is their domain management tool.
The account adminis also well integrated in a Joomla admin type GUI. Tech support is lightnin, and they have always been helpful for me.
In fact so long as you use fantastico etc for your installs they help out with Joomla constantly.
They are cost effective at $4.95 per month including domain name reg, unlimited emails, and any feature you could want.
One critixism would be I'd like them to load their servers with all preinstalled and available ((useful)) coms mods and bots.
While they are really leveraging open source well they have a host of other hard core solutions ((dedicated, VPS, Windows server..whatever).
Attached is the screen shot for user admin and cPanel