I find Wiwi's improvements to the out-of-date Stanz UI to be a must-have. If nothing else it shortens the time you need to spend in the loadout screen by a significant amount. The cooldown timers and alternate location for health/mana/stamina bars also make it much easier to manage your character's resources.
Now, I've never actually commed with the stock UI but with the changes this mod brings to the Comm screen, I can't imagine comming with the stock UI to be as easy and intuitive.
Oh yeah, almost forgot the biggest must-have mod in the whole game: Kill sounds from UT! What else is solely able to motivate you to go on a kill streak rampage?
I have the .zip file to replace the sounds, but I'm unable to find the thread about it on this forum... any help?