I've slowly worked my way down from a full-frame/APS-C system (Nikon D3 and D300) to a Pentax K5 and now to a Panasonic. The Nikons and K5 give better image quality and have more diverse systems, but the (almost literal) backbreaker for me was carrying the Nikons and the three f2.8 zooms and ancillary equipment in Iraq a couple of years ago: the weight was too much to deal with (although I'm older, in my 60s, so YMMV.) As it turned out, I never really needed the D3's low light capability as much as I thought I would, and for most of those shots, a Panasonic would have been okay, if not as good. You can wring a pretty damn good photo out of ISO 3200 with a GH2.
The K5 would have been fine for Iraq, even though it's not nearly as armored at the D3. I documented Middle Eastern archaeological digs with film N90s, in preference to F5s, because it was so damn hot that I just didn't want to carry the bigger cameras. It took three seasons to wreck the N90s, and we didn't treat them gently. I think the K5 would perform at least as well...in other words, I think the so-called "pro-build" of the big Nikon and Canons may be a little overdone. (But maybe not, depending on how much of a battering your cameras take. In Iraq, I was either flying or on military bases, and wasn't throwing my equipment around so much.) The K5 is not a hell of a lot larger than a GH2, though it is a bit larger. The problem is, the lenses are larger, except for the pancakes. Unfortunately, Pentaxes' pancakes don't make a full system, and the K5 does not have a flexible LCD, which I consider invaluable. But, it is a partial solution to the size/weight problem, and the high ISO quality is excellent.
The thing about the Panasonics is that you can carry a full line of lenses and three bodies in a briefcase-sized bag, and the GH's do have flexible LCDs. I use quite a small Kata backpack as my bag, and you can get it into the overhead even on small regional jets. I think the key thing about the Panasonics (and Olympuses) is simply size. You may not get quite the image quality of the APS-C and FF bodies, but for most purposes, it's fine. Most of the losses are at the margins, in low-light use or perhaps DR. If most of your shooting is in no worse than "poor" conditions, you should be okay; it's in "bad" conditions where you may have a problem. But again, for making the choice of an m4/3 camera, size is the key, not IQ. If the IQ is good enough for you, then the small size can be a great benefit. I used to work for newspapers as a reporter, and took occasional photographs for the paper, and I'd say that all of the m4/3 cameras would easily meet the requirements of newspaper and "typical" magazine shooting, except perhaps in sports, and the size/weight aspect would be a huge benefit.
If you really need maximum IQ in an armored body, well...you're gonna have to get a mule.
