Depends on how you look at it. If you see Evangelion as character-driven -- that the story is the story of the cast and that the resolution is their own resolutions throughout the series, it's actually a very good ending. People pretty much resolve their inner conflicts by the time the story ends, and most people are better off in the end despite being turned into Tang (Gendou being the notable exception). The alternative view is that of Evangelion as being plot-driven, that the storyis that of NERV attempting to destroy the Angels and the conflcit between Ikari and SEELE. In that sense, the ending is very bad -- everyone is now LCL, and yet individual identity remains, no side is the victor and all of mankind loses in the process.
At least, that's how I always saw it.