I just recently purchased the Momentums, and I have to say I enjoy them a lot. The sound is clear, albeit a bit lacking in presence and soundstage. The lows are slightly lacking, and the highs are there, but not as warm as Grados usually are (I have a pair of SR80is). Grados seem to be bright and warm to me, but lack substantial bass. The Sennheisers will probably give you a great well-rounded listening experience with each note, while the Grados might be better if you prefer classical, jazz, etc. At least, that is my experience. I love my SR80s, and I really love my new Momentums.
Hope that helps.