The base UT3 game was ok, nothing spectacular but it wasn't terrible in my opinion.
However, what 'killed it' (on the long term) for me was the ultimate lack of a very active modding community for it. There was one, there's always been some modding for UT3. But it just wasn't comparable to the kind of custom settings I played with on players-hosted (and modded) servers for either UT99 or UT2004, especially for UT2004. I believe that it was related to the engine, or the lack of editing tools that came super late from the devs or simply never really came out at all, and the community had to rely on third-party stuff that couldn't do much in the end. I don't remember the whole story on that portion of UT3's story (the modding one) but I definitely remember being 'stuck' with the vanilla game for the longest time until finally one day some basic mods started to come out, some custom maps (nothing crazy) and some custom character models. Then eventually some custom mutators got in and modified versions of the base game modes would pop up. I also remember that setting up some of those mods for UT3 was a real headache, and required a lot of manual INI and Coalesced files editing on my party and I just couldn't bother anymore. So yeah that part of UT3 just turned me off. However, the base game wasn't too bad, but I do think that it was the 'weakest' of the franchise.
My absolute favorites were some specific Invasion modes (with custom settings) for UT2004, with crazy monsters of all types and sizes (humanoids, too; anything from robots to aliens and even proportionally deformed models of Osama Bin Laden who kept making fart noises as he moved around). There was a lot of humor thrown in, hilarious one liners from some of those enemies (I recall a T-800 monster model with one Arnold voice line that kept repeating 'No way Jose' every couple of seconds), very unfitting yet super fun maps with over-sized objects or furniture (giant-sized living rooms, immense metro station with our own character models maybe 1/5th the size of a bench), lots of platforming and created amazing game-play moments especially with low gravity on. I used to play on one such server in particular (can't recall the name it's been too long) for a few months in a row almost on a daily basis, made some friends there (one of which I followed all the way to the release of UT3 and we played together for maybe the first two weeks or so), and the server was almost always full (located on the east coast I believe). That server added countless numbers of mutators and custom textures and enemies, music, visual effects. Also, both the players and the monsters would level up during waves (player levels would persist and monster levels would reset after each wave, but they could level up during any given wave). We had boss fights after each waves and the final two bosses required a lot of fast-paced quick thinking maneuvering and a crap load of strafing all while avoiding trash mods around.
On a side note I actually remember the name of the character I played with, and the character itself. My nickname was Tungsten and I played the (default green) Robot character Thorax ("Faster, stronger... better", will always remember that line). Ah... the memories! Damn, yeah... just thinking about all that again I get a big smile on my face. I had a crazy good time with UT2004, my personal favorite of the franchise (even though I adored UT1 / 99 on the PS2 specifically).