"OK, but not the best in the franchise"
While I am definitely a fan of the Unreal series, this latest installment is by no means the pinnacle of a great FPS action game-play experience. While it is an enjoyable romp for the first few hours, it quickly becomes obvious that, like most other titles in the Unreal series, that the single player missions really are only practice for your multi-player excursions wrapped in flashy-looking cutscenes and a flimsy "invading the planet, killing our race" storyline.
There are some great things to be said about Unreal Tournament III, such as the graphics, assortment of weapons, map variability and multi-player capabilities. The single player missions are where the game lets itself down, however. Yes, they've made an effort with some great-looking CGI cut scenes, but make no mistake, this is no Gears of War-type immersion. Rather, you choose each mission based on what 'type' of game you'd like to play (team deathmatch, capture the flag etc) and dive into play, completely oblivious as to the ultimate goal of the game. Vehicles do add a different element to the game but don't feel quite "right" to control, and we could probably have done without the superfluous hoverboard.
It wouldn't be fair to cover only the single-player aspect of UTIII, as Unreal Tournament is very well-known for its multi-player fun. A few friends and I decided to have a crack at the co-op Campaign mode, but only made it to the 3rd level on Normal difficulty (were we bad, or was the difficulty much too hard much too early? We decided to go with the latter for our pride's sake).
In short, the game is not great for it's single-player campaign, but does have some fun weapons to shoot and interesting maps to explore. Try the multi-player and you may be satisfied with what you find, but there are definitely some better titles out there. As mentioned earlier, the Gears of War series not only looks similar to UTIII, but has a more involving campaign and more interesting multi-player. Try UTIII if it's cheap, but don't expect too much from it.