Hey oldlamps, for what its worth, I've had M50's for a few years and love the sound.
I experience complete 'disappearing' speakers and killer soundstage all the time with quality recordings-- even more so with SACD and DVD-Audio.
I have them with a SVS Ultra 13 sub, and cross at 80Hz. Audyssey thinks they should be crossed at 40 or 60, but I think its a bit more natural sounding with 80Hz.
Who knows, maybe someday I'll decide to give my M50's to a needy person, and get M60's or M80's, but so far I've not had any urge whatsoever to want something new. I love the sound, and am extremely happy with them compared to all my friends' much more "expensive" brand speakers.