It's always tricky talking about buying camera gear. This becomes frequently a case of "Hey, please rob me!" or "Hey, look at me, I'm a douchebag with a well paying job" or other stuff. But since this story involves rage, my own stupidity, and a few thoughts about the whole freaking point of a lensless camera, I'll tell it.
All things being equal, a SLR is better. Especially once they get the sensors such that there's little phase-detection autofocus sensors built into the main sensor, so both the mirror-up live-view and the mirror-down SLR modes are equally capable. There is absolutely no potential for lag, excellent ease of focus, etc. On the other hand, mirrorless cameras are pretty darn good these days. With 120fps live-view displays, a lot of the lag issues disappear. My E-P3 has a bit of autofocus and focus issues because it's contrast-detection but since I'm not doing sports, it's actually perfectly quite fine for me.
Now, Olympus has a new SLR-styled mirrorless camera, which is partially a "halo" camera and partially a "sorry that you picked up all that expensive glass for the Four Thirds SLR lineup that's mostly useless" camera. It is very very close to the point where a serious action photographer will be fine using it. And I suspect that in the near-term future, most cameras are going to have on-sensor phase-detection autofocus sensors. The gap is shrinking.
However, removing the mirror and prism and other such parts from a camera only makes it mildly simpler. Mildly simple isn't really a great argument -- Saving some small amount of money by castrating the camera is not good.
The whole point of a mirrorless camera really ends up being that, by making everything much more compact, you can fit a really nice camera in your pocket. Plus, this really helps you fit in the sweet spot where you recognize that your fancy cellphone (iPhones especially) has a pretty damn good camera that's doing a pretty excellent job of replacing that compact digital you used to carry around for when you just wanted to take some casual shots.
As it turns out, I've got a bit of a divergent opinion. Apparently the SLR-styled mirrorless cameras are actually selling better than the rangefinder-styled mirrorless cameras in the US. So I've got some trepidation over happens in the next few years with cameras that I'd want to buy.
Unfortunately, I forgot to add my camera to my insurance. Which was dumb. And then, in the midst of a drunken evening involving pirate hats and enough rum to drown a pirate, I drop my camera while fumbling with other stuff. Which means that the only lens I owned at the time for my E-P3, the kit zoom lens, has a cracked lens mount.
The kit zoom lens was an OK lens. The really nice part about it is that it folds up. I wish more of the lenses for compact cameras folded up. And I'd intended to pick up some more lenses, but never got around to it. Because any follow-on lens is going to be a bit expensive.
But with no lenses at all (except for my Pinwide pinhole lens) I was free to consider what my options were. The Panasonic 20mm f/1.7 pancake was on my list the whole time, but I knew that wasn't enough.
On the other hand, I was not totally keen on getting the kit zoom lens again. The Panasonic kit zoom lens is better, optically speaking, but doesn't fold. The Panasonic 14-42mm F3.5-5.6 X had some questionable reviews. The Olympus 12-50mm F3.5-6.3 also had some questionable reviews and, after playing with it some months later, it just feels awkward. This left me with the Olympus 14-150mm f/4.0-5.6 as the last remaining option. It's not necessarily a pro-lens, but still a quality lens with a metal lens mount. It's just that, at it's smallest, it's still really quite huge.
What this means is that, after spending some money on OP\TECH neoprene sleeves, I was going to be leaving the large 14-150mm lens at home a lot of the time. However, I could just carry the camera and the 20mm lens in a nice little OP\TECH neoprene wrap. And, pretty much in the same way that I carry a 50mm lens on my 35mm film camera or the 90mm lens on my RB67, the zoom has always been fairly optional.
Thus, the primary joy-of-photography experience for the past year or so, where a bunch of non-photographically-oriented real-life details have conspired to limit my ability to really plan proper photographic trips and photo shoots, has been this setup.
If you want to do contrast-detection autofocus, you need a lens that can quickly track back-and-forth, without accuracy. This is how all of the Micro 4/3rds lenses are designed. If you want to do phase-detection autofocus, you need a lens that can accurately track to an exact position. These are exact opposite concerns.
What I'm kinda curious about, and we won't know likely for another year or two, is the long-term prospects for the lineup. Conventional wisdom is that phase-detection autofocus is unambiguously better... but I wouldn't be surprised that the final answer is not exclusively the use of phase-detection autofocus, either that contrast-detection mode and motors will continue to improve or that there's further advances in throwing a lot of processor power at a hybrid contrast/phase mode.