Amazing! I agree with Joe, in terms of pure musicianship they're probably the best band in the world.
My favourite album is "Scenes From a Memory (Metropolis Pt.2)", then like Wonkies said "Awake" and "Train of Thought" are also great. In fact, every album is a masterpiece.
It's prog, so it can get pretty wierd, but there's definitely something for everyone.
-Kev
P.S. A similar progressive rock/metal band is "Symphony X", slightly heavier, equal musicianship (They actually have a guitarist who can match John Petrucci, Micheal Romeo). If you like DT, check them out too.