One of my friends started to stream, and decided to have a Skype group call along with it.
He and some of my other friends immediately noticed the leap in quality from that old desk mic I was using to the MMX 300 mic. Seems like I'm louder and clearer, they don't hear my breathing even with the boom mic's proximity to my nose and mouth (I still keep it a few centimeters away from the foam cover's edge), and even accidentally bumping the mic boom doesn't result in annoying noise on their end.
I'll take that as a sign that the MMX 300's mic doesn't suck and Beyerdynamic didn't just cram any old mic onto a DT770. This is a good thing, since that mic's the only reason anyone would buy an MMX 300 over a DT770 and a ModMic.
Meanwhile, I've been listening to music with it directly driven from my X-Fi Titanium HD-equipped desktop and the integrated Realtek ALC269 on my Fujitsu T901 (yeah, I bought myself a new-to-me Tablet PC to replace my aging HP 2730p recently); it's still a pleasing listen, even without a dedicated amp in the chain.
Clamping force is moderate, enough to secure it on your head better than the MA900, but that also inherently means it's less comfortable over the long run. But as I said before, it doesn't feel like a painful vice on your head.
Sound leakage is practically nonexistent; I can take them off with them playing at listenable volumes for my tastes, not too quiet, not too loud, and NOT hear anything whatsoever even without the headset being put on anything to close off the earcups. If you don't want to disturb the person next to you with your noise, this is your set!
On the flip side, that person can still disturb YOU with his noise, if it's loud enough. As stated before, the isolation's only moderate. You've probably got better options for shutting out the outside world...but you'll likely lose out in comfort, sound quality, and relations with anyone who needs your attention.
Now, as for a niche thing that only concerns PC simulation gamers...I've found that the TrackClip Pro for my TrackIR doesn't really fit on the MMX 300 headband well, especially since I have the cups only extended a notch each for my ideal fit. Seems like the best place to fit it is on the black part just before the headband padding, and even then, it BARELY fits.