Good question. I freely admit that I'm a neophyte when it comes to high-end head-fi; I'd say I have a somewhat discerning ear but certainly not a trained one--I know enough to be able to tell the difference between sound that is warm, dark, muddy, bloated, dry, detailed, etc., but certainly not with the kind of precision or authority that some long-time head-fi'ers here on the board seem to be able to discern or describe.
With that said, there's no question that the HD800s shine and sparkle and reveal every detail in the source material, but personally I just find the PM-1s more fun to listen to. Some would likely accuse me of having an un/under-developed sense of appreciation for what I'm hearing, or that I'm not necessarily hearing the whole story when I spin up the PM-1s over the 800s--that may certainly be true. Sad to say, perhaps I'm simply not sensitive to all the things I'm supposed to be getting out of the 800s in the first place. For example, I can certainly hear the wide soundstage everyone raves about in the 800s, but I don't find that the narrower soundstage of the PM-1s really detracts from my enjoyment of the underlying tracks.
Certainly my musical tastes and listening environment don't lend themselves well to a bout of serious, critical listening to a bunch of high-rez audio files in the first place. But that's okay--I bought the HD800s in large part to see what all of the fuss was about and whether I could really tell there was that much of a difference over other offerings. And certainly there IS a huge difference in performance...but I tend to reach for a stack of CDs or my few SACDs/DVD-A titles more often than not just to have fun and get lost in the music, but not necessarily to critically dissect a recording or focus on every detail or nuance present in the bitstream which I get the sense (perhaps narrowly or naively so) is where the 800s are really more in their element. Again, maybe I'm missing the point or don't yet recognize it when it see it...
And...this might be a bit silly...I know lots of people gush about how comfortable they are, but in truth, I find the massive elephant-ear size of the 800s a bit disconcerting because there's SO much empty space between the pads and my ears. Perhaps I just can't get past what my brain tells me headphones are supposed to feel like on the head, but I routinely catch myself pulling off the 800s because I've had enough physically (not emotionally, not because the sound is fatiguing) and just want to sink into something that, to me, feels a bit more inviting and comfy.