I just got my UM2s today. After listening to nothing but my ER-6s for months, the change is a total shock. I have to get used to the lack of treble. I have to EQ the hell out of them to get proper treble AND bass out of them. I was considering returning them for the Super.fi 5 Pros, but apparently those have less treble than the UM2s. I could get some serious thumping out of my ER-6s with the proper equalization, so I may check out the ER-4Ps...
Anyways, yeah, I think I'd rather have full-sized headphones for most listening, but since I'm a college student on a budget, proper IEM choice means I need no amp (the 1212m is a crazy powerful source!) and they blocks out roommates.