I got mine last week, so in terms of burn-in it's still a fledgling pair. I use these massive things for "portable" use--basically the purpose of buying these was to block out some of the commute noise that comes with subways.
I like them a lot driven from my iPod nano. These are my first pair of closed full-sized cans. They're very laid back next to my reference points (SR225 and HE-5), and I do appreciate the presentation, which I found wasn't so laid back as to make Rock its "weak genre".
For me, it's hard for me to pick out an altogether favorite headphone from my very small collection. If someone asked me to pick the ones that produce the best sound, it's easy to say the HE-5. But I can't bring my HE-5's around, and those things can't really play very well from an iPod. What I'm trying to say is I think there's plenty for me to still enjoy out of the 840 in spite of having heard a phone that is a vertical upgrade (HE-5) and another phone that is in a many ways a lateral "upgrade" (SR225). So, while I will not dispute the claims of some people who didn't like this phone, I think for $130 this was well worth my money. Of course for ppl who looooove the Grado sound or any other signature to the point where personal preference leaves little room for tolerance, then yeah... these might not be for them.
My favorite part about these headphones is their seeming durability. I haven't put these to the test of time as many others have, but after owning the HE-5 for over a month, I'm glad to have a pair that I can "rough up" a little more.