Interesting. I just got the PM3s two weeks ago. Spent the first week burning them in. And so, I've been using them exclusively this week, attempting to evaluate them. My findings by-and-large reflect your thoughts.
Weight and clamp force? Yeah, I don't have any issues with them. I've never had many over-ear headphones in the past and instead, my "larger" phones have typically been on-ear. Those types of phones (at least, the ones I've bought in the past) have had a lot more clamping force. So the PM3 is nice and relaxed, as far as I'm concerned.
Sound stage? Yeah, it's okay. It's actually very similar to my IE800 IEMs. It's not super wide or deep. But at least music doesn't sound as though it is exclusively occurring inside my skull.
Bass? This is probably my biggest concern. It's...rather thin for my liking. Not too much impact.
And I will absolutely agree with you about the material used for the pads. The two headphones that I've had in the past that used synthetic "leather" have broken down in just a couple of years of use. So I'm a little leery about the longevity of them on the PM3.
The other day, when it was relatively warm, after a roughly 1-hour session with them, I was surprised to see beads of sweat all along the pads once I removed them. I wasn't sweating. But the heat that the PM3s collected resulted in sweat forming right along the seal. Well...at least they'll be nice to use in winter.
Instrument separation and detail is good. Although, after using my IE800s for the first time after about a week of exclusive PM3 usage, I was struck by how much more sharp, clear and detailed they are compared to the PM3. I'd say the PM3 is more "smooth" and laid back compared to the IE800. Thankfully, the PM3 doesn't sound veiled or muffled at all to me—which is something I was worried about.
Overall, I'd say they're a solidly good set of full-sized phones. Especially for something that can be driven by portable sources.