It's very simple. I've never had a problem with a one-click install, but there is one important step that many overlook: the destination directory must be empty (including hidden files like .htaccess) or the process will fail. No big deal though, just delete what needs deleting and do it again.
One-click updates are also available. I'm not sure about the other programs, but with Wordpress, I prefer to upgrade manually.
The one-click update basically does a new installation, but puts your old one in a .old directory. No big deal if you use a plain installation (no themes, mods, etc...), but if you do, you'll have to either re-install them, or copy those specific files back over from the old directory. I prefer to just overwrite the files that actually changed, leaving any customizations intact.
Also, if you're coming from a CPanel/Fantastico host, it seemed to me that Dreamhost is quicker at updating the packages. Unless something has changed... it's been a little while since I've dealt with Fantastico.
Also note that you're not limited to the one-clicks. If there's a different CMS, cart, forum, etc... you can always install it manually.
We're still talking about the one-clicks, right? They're PHP, but as far as configuring goes, that's usually handled by the installer asking you the questions.
The panel also notes if a package requires PHP 4, PHP 5, or either. You can switch from 4 to 5 on a per-domain basis at any time in the domain management section of the panel.
Save up to $96 at Dreamhost with ALMOST97 promo code (I get $1).
Or save $97 with THEFULL97.