UM2's are a tiny bit sibilant, but only with recordings where it is already a problem. They don't add any sibilance to the music, but they don't conceal it either by softening up the treble, which for example is what the SR-001 does. So, if you have a lot of poorly produced rock and metal around, then sibilance could potentially be a problem - but if not, then it shouldn't even be noticeable.
I like the UM2's, but I won't go as far as saying that they can match high-end full-size headphones. They're nowhere near my SR-404 rig. But, they're good enough that I can use them portably and not care about their sonic deficiencies. They have a nice, smooth, musical sound, slightly upfront, with a solid palpable bass, though slightly muddy. They may not be as detailed as the ER-4P/S but they are certainly very detailed. And on top of that, they have a thoughful ergonomic design.
I think you'll be happy. Just make sure to try different kinds of tips with them. I use Ety tri-flange turned bi-flange, but no two ear shapes are alike, and what works for me, may not give you the best sound. Experiment around before you make a final judgement.