This game was basically overlooked when it came out because it was realeased at about the same time as Quake2 which was itself a good game. Unreal however has one distinct feature that has never existed in a BSP based game (Quake 1,2+3, Half life etc)....
Massive open areas.
The areas on unreal include massive canyons, complex acient temples, spaceships, entire villages and underground mining facilities.
The playability of the game should not be underestimated, it is great and the few flaws of the first game have been removed. The monsters, especially skaarj warriors are very intelligent. Their AI is incredible, they will try to dogde every bullet and missile.
The story is well worth a mention too. You play a criminal, done for god knows what, on a prison ship travelling through space, but something goes wrong and the ship is pulled to a mysterious planet where it crashes..
You come to sometime later, the ship is nearly empty but screams echo round the steel corridors and rooms. You grab a gun from a recently killed guy and go adventuring on the planet. You meet the nali, a banal race of four armed aliens who's religion prevents them from combat. They are being ruled by the Skaarj, a race of vicious creatures from another planet who force the nali into slave labour and despise all other forms of life. You need to get off this planet alive, and you've got to get guns, lots of guns. You've got to have wits, for the complex puzzels, and a keen eye to spot clues and secrets.
This game will keep you entertained for hours, perhaps days, and you still wont complete it if you just go around shooting everything. It is a First Person Shooter, but like most FPS's it's violence factor is over stated.
If you're looking for Adventure, look for this. If you're looking for violence go buy Kingpin