Other than being visually stunning, Unreal Tournament 3 doesn't offer much improvement to the previous iterations or the First Person Shooter genre.
If you've played Unreal Tournament 2004, then you already know what to expect, hardcore intensely twitchy first person action. The only additions to the game is the new Warfare mode which, is the same concept as UT2k4's Onslaught mode except now there are extra support nodes that spawn extra vehicles or forward spawn points. For people who don't know Onslaught and Warfare gameplay is where a team must capture territorial nodes that link to other nodes that become unlocked. Each team continues capturing nodes until a link from one team's base connects to the other, then the winning team can start attacking the losing team's core node. Other than Warfare, UT3 supports the traditional First Person Shooter modes like deathmatch, team deathmatch, and capture the flag.
The game runs surprisingly well on mid range computers and look extremely well at that. I own a 3.4Ghz Pentium 4 computer 2 gigs of ram and an Nvidia 7900 GS and I get around 40 frames per second. Although these framerates are not suitable for hardcore tournament play, they are just fine for casual play, but Unreal Tournament's base is composed of hardcore twitch gamers so to get the most out of the game you'll need a pretty powerful system.
If you're not a hardcore Unreal tournament enthusiast or a hardcore twitch gamer, or don't have a powerful computer, then I suggest against buying the game, otherwise, go for it... the tried and true core First Person Shooter elements are there, its up to you to enjoy it!