I found United Offensive (When I saw "UO," I thought this was the Ultima Online
thread) exciting, but way too difficult for my tastes, and I never finished it.
Don't forget that United Offensive was by Gray Matter Interactive (makers of RTCW's single player game, not its generally more well regarded multiplayer component), not Infinity Ward.
That may make me a wuss, but I'm generally wary of "follow up military shooter expansions" that generally aren't handled (sometimes for time reasons, but sometimes because the original developers had moved on to other things) by the original dev. So I didn't enjoy either Medal of Honor: Alllied Assault's two retail expansions very much, as both seemed too brutally difficult and not as well paced (imho) as the source product.
However, if you found COD a cakewalk, then by all means you may find UO more to your tastes.