Not as good as other UT games, and UT Veterans maybe disappointed, but that doesn't make it any less fun.
One thing about the game that was done well with other UT games was the weapons. Not that the weapons in this game are bad, there not, they feel awkward when compared to the other games. But they all balanced well and it will come down to preference. I like the Flak Cannon (shot gun like weapon) and when you manage to get a Enforcer (Small Pistol) from a downed foe giving you two guns you can really do some damage.
The game mode in UT3 are not as varied as they are in UT 2004, but the ones that were removed weren't really played by anyone. But one thing that really upset me was the lack of the Invasion Mode. Invasion was a game mode in which you and your team struggle to survive wave after wave of monsters. On it's own invasion was nothing special but their was a Mod that changed Invasion to RPG Invasion which is my favorite game mode!
UT3 has standard game modes, Deathmatch, CTF, nothing all that new. But the warfare mode is the game mode your more likely going to find yourself playing. Warfare is a lot like Onslaught in UT 2004, each team creates nodes to like to the other teams Power Core and destroy it. In this game that same task can be achieved in many different ways. Plus with all the vehicles and doo-dads you have plenty of fun with you and 32 other players.
Single player is just to get you used the game, if you want the true experience you have to play online. Anyways A.I. in this game can be a challenge as long as you bump it up other then that it's like shooting fish in a barrel. On Single player Warfare can be annoying as teammates can be pretty stupid and not do the right thing, for instance the map Tank Crossing is a map where you need to get you tank to the enemies side to destroy the Power Core, Your teammate might up in the tank and get stuck in a ditch and you'll have to wait until they somehow manage to get out a year later or the enemy blows up the tank.
With all that said UT3 is a excellent game and if your bored of game that rely on tactics and stealth and just want to blow your enemies to tiny bloody pieces this game is for you.