I used UI mods when the game first came out because I didn't really have a choice. Playing a Warlock past level 20 without extra toolbars was completely impossible a few months ago; my choices were to pick a class with simpler toolbars (Rogue), quit the game or download a Custom UI.
I used to have a combination of Cosmos and Gypsy, and they were pretty cool. They added a ton of features that were incredibly useful to my first two characters (the aforementioned Warlock and a Priest)...but of course a lot of other crap I didn't need. This is the inherent evil of Cosmos in particular: in an effort to have every feature under the sun, it comes across as a little excessive.
I quit the game for a few months and only just returned to it a couple weeks ago. When I quit Blizzard was going to great lengths to accomodate users who didn't want to use the Custom UIs and deal with the hassle of reinstalling/reconfiguring them with every new patch, and they were doing a fairly decent job of it by providing the most essential features these other mods had, like extra toolbars and buff timers. They weren't quite there yet, though. Between me quitting the game and returning to it, however, I'd say Blizzard has achieved this goal to the point where I just won't bother with the Custom UIs anymore. I liked the way Gypsy made my interface look, and if I still used a Priest there are some features that would be essential, but as a Warrior on a PvP server I don't need a lot of clutter on my screen anyway, and with most of the nice Custom UI features in the default UI, I'm not going to bother with them anymore.
The only feature I really miss is the quick read quest text option; it's so nice to just be able to skip the slow rolling text before you accept a quest. Still, there are certain areas of the game where this could definitely be considered cheating, as far as I'm concerned, so I'm not sure I'd want it anymore anyway.