Technically brilliant, incredibly dull.
Whilst Unreal was technically brilliant, it just didn't excite me as a game to play. Yes, the colour palette was wide and varied but it lacked an artistic style. The designers spent more time trying to exhibit the technical feats of Tim Sweeney. I much preferred the feeling of the environments created by the guys at Id Sofware, even if Quake was all very brown. And I also preferred fighting Steve Polge's Reaper Bot for Quake deathmatch, than the opponents in Unreal. Admittedly, it's a different style of gameplay but I don't think Epic really used his talent to Unreal Tournament 2003 which was a lot more fun.
It also disappointed me that both Quake and Unreal lacked the multitude of on-screen opponents that you could fight in Doom. Yes, they were proper 6DOF 3D, but they lacked the excitement and fun of Doom. Not to mention that neither of them had a multiplayer co-op. Had to wait for Left For Dead to get something of a similar ilk.
And I think as other people have mentioned, the best single player story led FPS by a country mile was Half Life.