The slight touch of harmonic distortion from a tube amp can sound nice, even though it might not be totally transparent. Tube amps are also a lot of fun to build and tinker with because of the huge diversity in tube models, the operating point at which the tubes are run, the gain stage topology, power supply designs, etc. Personally I find that different tube designs almost always sound a little different from each other while well-designed transistor amps sound indistinguishable or almost indistinguishable from one another.

Pete Millett, who runs a DIY Tube site, made a good presentation on this: http://www.pmillett.com/etf_sod.htm