A brief history: When the first Micro 4/3rds camera came out, I didn't buy it, but I knew right away where things were going. I waited through the earliest bodies, all of which were SLR-styled and not especially compact. Then, eventually the Digital PEN mockup came out, followed by the E-P1. I waited through till the E-P3 came out and got one, because it was good enough.
After the E-P5, Olympus released the "OM-D" series of SLR-styled cameras, starting with the E-M5.The E-M5 has been a huge hit, as far as Olympus cameras go. It had a bunch of new nice features, plus they were really able to style the camera right. It looks like a camera ought to look... not slavishly retro, but not totally modern. And it's tiny, so, while it's still got the hump, the whole works is much smaller.
I still like my E-P3 and it's still a great tiny camera, but the E-M5 was interesting, largely because it had the "Live Time" mode that displayed updates as a long exposure photo was being taken, which is incredible. The E-M1 had the same thing, but it added WiFi control, except that a lot of folks were annoyed with that sensor's long exposure noise.
Eventually the E-M5 Mark II came out, which added a few of the E-M1's features back. Unfortunately, it didn't add phase-detection autofocus, that's about the only thing it really left out. But it did add WiFi, so I can connect to it via my phone or iPad.
The growth of digital camera functionality has slowed sufficiently that I tend to keep the E-P3 and the 20mm pancake as my "everyday carry" camera. I have a set of stuff that either lives in my bike's panniers or my backpack, depending on how I'm getting around, and that's pretty much my work laptop, headphones, my bag of pens and cables, and my E-P3.
Conversely, when I'm doing serious photography, the E-M5 Mark II is what I'll have with me and I'll leave the E-P3 at home.
I will say that if your camera is the same vintage as an E-P3 or older, it's most certainly an update that you will notice.
It turns out that, for the most part, the real impressive growth of real camera features has slowed up quite a bit. The E-P3 does pretty well at high-ISOs, focuses pretty darn fast, etc. By comparison, the E-M5 Mark II focuses faster, definitely has a sharper picture at high-ISOs, and also is a bit bigger. It's really nice. But it's not a killer feature.
There is, however, one extremely killer combination of features that my E-M5 Mark II has that makes my life easier: I can do light-paintings with the live-view mode over WiFi with my iPad. This means that I set up the camera on the tripod, lean my iPad against the tripod, push the shutter button on my iPad to open the shutter, watch the light painting take shape on the iPad, and then press the shutter button on my iPad to close the shutter.
It also has a GPS Synchronization feature, but it's flakey. The last two times I used it, I recorded a GPS log but didn't get any of the images to be geotagged.
There's more buttons on the E-M5 Mk II than on my E-P3. That's handy for serious shooting and marginal the rest of the time. Truth be told, Program mode on the E-M5 Mk II is really good so, while I'd keep my A95 or G7 in Av or Tv modes, on the E-M5, I just leave it in P mode and shift the aperture up and down as necessary.
I've found, while switching, that I tend to put the E-M5 Mk II against my eye, but when I switch back to my E-P3, I'm not suddenly missing the EVF.
There's a switch that they added that lets me switch the two control knobs between Shutter and Aperture and ISO and White Balance. Because the latest auto-exposure logic really does make Program mode super-usable, I'm mostly making a value judgement about bumping ISO when necessary and the switch makes that easy.
The only problem here is that I've accidentally changed the White Balance on occasion, although that's not always a huge problem because I always shoot RAW.
When dealing with a tripod, you need to prevent the camera from rotating against the mounting plate. With a heavy lens and the camera on the side, there will be rotation that you'll notice in a long exposure. Solving this turns out to be a bit hard. They tried cork and rubber but that only works sometimes.
A long time ago, noises were made about the Arca-Swiss style of quick-release. There was a really good ballhead by Arca-Swiss already, so this company called Really Right Stuff decided to make really good quick-release adapters for the ballhead. And by "really right" what they meant was to carefully measure the camera body and any available holes or edges or perturbations and figure out how to make a quick-release plate that mechanically latched to the camera body while not blocking anything useful.
If the Arca-Swiss mount hadn't come along, the right answer might have been to standardize a pair of support holes around the tripod socket, but at this point, it's pretty much the primary mount for stills.
Olympus has an accessory grip for the E-M5 Mark II, the ECG-2, that grips the body and also has the Arca-Swiss plate machined into it. Plus, it thickens up the body around the shutter button for an easier grip.
I realized that there were some clones of it online already, so I got one of them.
Let's be clear here... camera development has slowed. Olympus and Panasonic got astonishingly lucky to be first with the Micro 4/3rds mount and were also lucky to get a good set of lenses fast enough to be a contender. The Samsung NX and Nikon One systems both ended up dying recently.
On the other hand, Olympus and Panasonic's financial health for just their camera business is a bit shakey. And there's always the risk for them that Canon will get their act together on the mirrorless sector.
Still, I expect to see an E-M5 Mark III with incremental goodness instead of a completely new, all different, revolutionary camera.
I'm still looking for a long-term follow-on for my E-P3. I've been eyeing the alternatives and, for where I am right now, the PEN-F isn't really that interesting compared to going really low-end on the E-PL8 or the Panasonix GX80/GX85.