I'm just recovering from a long trip, and thought I thought I'd add my field experience with the GH1.
I hiked from Erfurt, Germany to Rome (because Martin Luther did 500 years ago) between August 22nd and October 29th this year (see it blogged at www.hereiwalk.org). I took almost 12,000 pictures along the way. You can see my photostream at http://www.flickr.com/photos/hereiwalk/collections/72157624330836765/ It's clearly capable of taking great photos.
First of all, it is light for it's class. With the 14-140 lens, it's about two pounds (3/5ths of that the lens). With the 20mm, it's not much more than a pound.
I chose the GH1 because of it's video ability; we are making a documentary of our trip as well. The raw footage it produces really can't be beat among consumer cameras, especially if you run a firmware hack. You have to keep to the 14-140 lens, however, unless you shoot with a separate mic; anything but a linear motor produces audible noise while auto-focusing during the footage. I used the 20mm, but only in scenes when I didn't need to change focus.
As a stills camera it's quite good. It seems flimsy, and though I never dropped it, it held up to a bit of grit and way too much rain, and being generally bumped around. I kept it in a padded case when I wasn't carrying it.
Battery life is pretty good. I got about 400+ shots off (mostly with the EVF) before it pooped out, including a couple of dozen short film clips.
I shot mostly with the 14-140, which I have to say I'm very happy to have had. It's pretty tough. The 10x range fits most travel situations, and I never had to swap lenses during the day--unless while I was inside a dark church. It's a dark lens and suitable mostly to daylight. But as the quality of high-iso shots increases, it will become more useful as I graduate to other m4/3s bodies. It's not uber-sharp, especially at ultra wide and super tele, but about as good as superzooms get; perfectly fine for full-page blow-ups. At portrait lengths, it can be as sharp as any 4/3s lens.
I did have a strange problem with the aperture sticking wide open when in manual modes. This wasn't a problem (5.6 isn't very open) most of the time, but very frustrating at others. Usually when I bumped (or hit, rather) the lens, it unstuck. After coming home I sent it in and they exchanged the lens for a refurbished one.
The multiple aspect sensor is a real plus. No cropping and you get full coverage for 16:9, 3:2, 4:3, and 1:1.
The electronic viewfinder is absolutely excellent. I was shopping around for an "upgrade" and found that the DSLRs, unless you get into the professional Canons or Nikons, have very dim, small viewfinders, compared with the GH1's. It's not up to retina display clarity, but close. And quite bright, even in dim situations.
The fully articulated screen is incredibly useful (and quite bright), not only for video (for which it's an almost necessity), but for candid shots. It will change the way you shoot people.
The contrast-detect AF worked great, and is as fast as other dSLRs. But it struggles with non-isolated moving subjects. What doesn't. It struggled to keep on flying birds, too. As with most autofocus systems, you do have to choose the right mode for it to work properly; with the decreased depth of field of m4/3, it's important to get this correct.
As usual, macro work requires manual focus a lot of the time. This works well, but is a bit tedious to use on a regular basis. The 14-140 is an OK macro lens when extended, but at 140mm, the minimum f-stop is 5.6; which means high-ISO shots. I drool for the 45mm Leica 2.8 ($900!).
The main complaint I have is rather poor high-ISO. At 800, it was useable. But at 1600, it shows banding in certain scenes. I tried to use the 20mm/1.7 indoors, but that was not always possible.
The jpegs out of camera take a lot of tweaking in the camera menus to get right. Too much green/blue; agressive noise reduction, etc. Plus, as you know, no camera does that well at AWB. I just forwent them and used all RAW; I couldn't be happier.
Although this camera poses as easy to use with the iA functionality, it takes some know-how and potentially pricey PP-software (I use Lightroom) to get it to really sing.
I'm very happy with the GH1 as a travel/hiking camera, and am saving up for more lenses. I'll probably skip the GH2, and pick up the GH3 body when it comes around.