I enjoyed the se215's for over 2 years. About 2 months ago I bought my HD600's. I agree that they are both great value's, but the se215 are a great value $100 IEM and the HD600 are great value $350 open back headphones. So, I can't really put the se215 in the same league, sound-wise, as the HD600. To me, the HD600 is audiophile quality gear, the se215 is a nice sounding $100 product that probably sounds better than $100 worth, but to me doesn't approach audiophile sound quality (whatever that means).
That said, I also recently picked up a pair of IE80's and those are a definite step increase in sound quality from the se215. While the sound flavor of the ie80's is very different from the hd600 (though I feel the se215 also sound different) without getting into detailed comps of each, I do feel the sound experience in general terms of sound quality of these two products is comparable.