I got into m43 initially for video and infra-red, and have stayed in m43 for the image stabilzation. I own an E-PL1 (converted for IR use), and E-M5, and an E-M5ii. I am perhaps a bit more lukewarm toward m43 than other posters. I'm well aware of m43's limitations in comparison to my K-5iis and my Pentax glass. But I've stuck with m43 because of the industry leading IBIS in the E-M5 cameras. With my K-5iis I find I need to shoot on a tripod to get the sharpness I want. The three axis stabilization is not good at longer distances frequently found in landscape photography. When I don't wish to drag a tripod around with me, I reach for one of my E-M5's. At the wide-angle focal lengths where I normally shoot, I can reliably get tack sharp shots (just as good as if they were shot on a tripod) at 1/10 a second, and sometimes even half a second will work. (However at longer focal lengths [FFE 100mm+], I don't really notice much difference between Pentax and Oly IBIS.)
In terms of image quality, while I wouldn't say there's a huge difference between Pentax and Oly raw files, it's definitely not insignificant --- at least to my eye. And it's not just the ISO and the DR. My Pentax RAW files consistently produce richer color and Pentax lenses provide more tactile rendering and consistently produce more beautiful files. Top m43 glass is very sharp and contrasty. My Oly 17/1.8 may be a tad sharper than my DA 21; and my Oly 12/2 is sharper toward the edges, with less field curvature, than my DA 15. But images from the Pentax limiteds just look better: richer color signature, better rendering. As impressive as top Oly glass definitely can be, it's impressive in a generic sort of way. Unlike some of the other posters, I'm not impressed with m43 high ISO. DR and ISO of the E-M5 is very similar to my old K200D, except the Pentax DSLR produced richer, better color. Of course evaluation of ISO is partly subjective. Some people are fine with what m43 can do at ISO 1600 and higher, some are not. I'm in the latter group. I rarely shoot above ISO 200, and am rarely pleased with what I see at higher ISOs. In landscape photos, skies are noisy at ISO 400. Above ISO 400 contrast and saturation take a hit. When the Oly 40-150 f2.8 came out, I considered it as a zoo lens. But because I was unhappy with higher ISO shots on the E-M5, I bought the Tamron 70-200 f2.8 instead. I'm perfectly happy with ISO 1600 on my K-5iis. No loss of contrast or saturation, very little splotchiness, and the noise tends to be very fine grain and palatable. I know I'm probably being quite a bit fussier about high ISO performance than most photographers would be; but some of my zoo pictures taken at ISO 800 have wound up as bus adds, so quality is important to me.
The new E-M1mk2 is supposed to have significantly improved ISO performance. It scores higher on ISO measurements over at DXOmark than the K-5iis. Of course, there are measurements and there is real world output, and the two don't necessarily enjoy a 100% correlation.
I find the E-M5 series of cameras are great for hand-held photography and are fun to use --- especially when matched with light high quality primes --- but when I when I'm looking for very best image quality, I always reach for my K-5iis, not my m43 stuff.
Shot taken with E-M5ii and Oly 17/1.8: