I just received mine today. Initial impressions are: It's very tiny; smaller than a 2nd gen nano. It has a slight turn on pop which I've been told they are fixing in v2. Wow, this thing sounds pretty good. It doesn't just boost bass but boosts the entire frequency, so that mids and highs don't get lost in the increasing bass. Sounds good with my W4's; with one to two notches of bass from flat, they hit harder in mid and sub bass, while keeping the cymbal crashes loud and clear. Very fun and seems to be pretty darn clean on my first few minutes of listening.
I'll have to spend more time with it to tell how good it really is but for IEM's I don't see a reason to by a regular amp over this thing. I'm really surprised this hasn't taken off to FOTM status on head-fi.