I've been listening to these for about 4 weeks now and they have really grown on me. I've never been a fan of any Bose product, and got these kind of by accident.
Normally I listen to Sennheiser PX100's or Westone UM2's on my iPod, but I am really enjoying the Bose in ear's and they are currently my favorite option. For the following reasons:
Comfort - the most comfortable phones I own. They just rest snugly in lower bowl of the ear, not plugging it up like typical IEM's, nor pressing the head or the ears like circumaural or on-ear phones. Also much softer than standard ear buds.
I can hear the outside world - I used to think that was a negative, but since I am most often listening to the iPod on the go, I've discovered that being completely aurally cut off from the outside world is sort of unnerving. I prefer a little outside noise.
The sound - I don't think they are as accurate as my Senn's or UM2's, but they are incomparably better than any earbuds I've heard and very pleasant to listen to. At very low volumes they are surprisingly balanced. Also, and this may have something to do with the large vent in the Boses, they don't seem to pound my eardrums as much as other phones at similar volumes. The sound is also peculiarly open, more "outside my head" than my other phones. They sound as open as they feel.
All in all, they are just a really fun set of phones. I think Bose's idea of a design midway between a bud and an IEM is a great one.
Have to get ahold of a pair of these new-improved silicone tips, though. Mine have fallen off 3 times which is 3 times too many.
An important note about fit: if, like me, you tend to use the largest size tips on a typical IEM, don't assume that the large tips for the Bose will be the best fit. These phones don't sound good with a complete seal: that just makes them all bottom end, no treble. Try each set of tips to see what makes them sound/feel best to you. For whatever reason, the mediums worked best for me.
Also on my iPod, they sound best with the treble boost setting (as do my UM2's).