I've been an Olympus user since around 1990, starting with an OM-1N. I'd also had an IS-20DLX, D-300L (one of the first two Olympus digital cameras), C-2500L, E-1, E-5, and most recently, the E-M1. I've shot a lot of high school sports with the E-1, E-5, and GH3 and skate park sports with the E-M1, Panasonic GH4, and GX8.
I attended an event to try a pre-production E-M1 and various lenses. I wasn't happy with the compromise. If you used the E-5 and was suddenly pushed to use the E-M1, you wouldn't be happy, either. Thankfully, these things are optional.
The E-M1 has a tiny battery capacity--far less than the E-1/E-5's or GH3/GH4's capacity. Add to that an electronic viewfinder that is always enabled, and the rear display that is accidentally activated too easily, and you have a requirement for several batteries per day. I have 3 batteries for the E-M1 versus 2 batteries for the GH4.
The E-M1 Mk II has a larger capacity battery and a reworked grip, since that is where the battery resides. (The external battery grip supposedly takes only one battery.) This has a lot to do with 4K video recording but also with the enhanced auto focus processing.
The enhanced AF processing is important. They're dedicating a quad core processor for auto focus. They also claim 121 cross-type phase detection points and 121 contrast detection points. That could help greatly. Will that be as good as a dSLR? Olympus will try to make it work. I changed my methods to get focused shots, instead of depending on continuous/predictive AF and tracking that didn't really work.
On the other hand, Panasonic claimed that they didn't need phase detection pixels at all for the GH4, but their AF is a headache for me. When I think I've got something in focus (sports don't slow down), I end up with something much larger in the background in focus far too often.
The E-M1 viewfinder was fairly good at 0.71x of 135 Format. The Mark II viewfinder is supposedly sized at 1.48x and runs at 120 fps, which seems to be double the refresh rate. The magnification is incredibly big, but the camera body isn't out and we may find that it isn't quite like that in real life. Update: it's actually 0.74x apparently--slightly larger and/or more magnified than the original E-M1 viewfinder.
Increasing the mechanical shutter's maximum burst rate from 10 fps to 15 fps seems exceptional, but for me, the 200,000 shutter actuations (as good as the GH4) hits home. I exceeded the 150,000 actuations with my E-M1 and had to have the shutter replaced. At 10 fps, photos can be created very quickly and at 16MP, they take quite a bit of space. At 15 fps and 20MP, that just makes things more intense.
The enhanced in-body image stabilization is rated at 5.5 stops, which allows the hi-rez 50 MP mode, good for still scenes. Supposedly, the mode can handle some gentle motion. Hopefully, it's as good as that of the latest Pentax bodies.
It looks like they've addressed most of my concerns, including the single SD Card slot, to make the E-M1 Mk II more like the E-5 successor that I expected three years earlier. Then again, the E-5 was about 3 years late.
Of course, the Panasonic GH5 is equally compelling but for different reasons. I wish I could replace my E-M1 and GH4 right now.
Update 2016.09.24: All of the previews I've seen so far seem as though the E-M1 Mk II is better than anything else near its size or specifications. Don't put the marketingspeak in front of me, though. I doubt salespeople from a company. I'd love for it to be so amazing, but it has to work that way in my hands for me to believe it.
Update 2016.11.01: Okay, at US$1999.99, I'm not quite as ready. If it had been $1699.99, as I had expected--E-1, E-5 price--it wouldn't have been quite as off-putting.
I'm sure that they can't get the image quality out of it that is worthy of that price. That said, if I was still making money and needed one, I would pay for it at full price, perhaps with a little resentment.
Is the newer sensor with 121 AF points that good that it is worth the extra US$300?
Of course, I can just wait to see what happens--anyone can--and I believe at this price that a lot of anyones will be waiting to see.
Update 2016.11.10: I'm amused to see some comments about the body. I swear that you could take photos from a Nikon D810, a Sony A7R Mk II, and a Canon 1Dx Mk II, crop them to a 4:3 ratio and some people would complain about the photos' quality because of the "tiny/toy sensor".
I suspect a lot of people know very little and judge only on hearsay. Did Ansel Adams brag about the size of his film?
About 10 years ago, I asked people why they only considered Nikon or Canon--they responded with something like "oh, my mother's ex-roommate's brother's friend's uncle uses one." and I would laugh because they didn't try anything and they didn't research anything but it was "clearly the best".
A lot of the comments I see relate to slow and casual photography--flowers and such. Yes, a used Sony A7 will probably outdo the E-M1 Mk II, with the correct lens.
However, working at 10 or 12 fps, micro Four-Thirds excels. I will admit that my heart defect gets in the way now, but it's very easy to use the E-M1 or GH4 to get good, and occasionally great, skate park shots. When I could move easily, there wasn't anything else that could keep up with me. The speed of the focusing has a lot to do with it and the stepper motors used are often extremely fast, especially in the fixed maximum aperture lenses. This also helps with video, of course.
From what I've seen of samples, the E-M1 Mk II outputs much better ISO 3200 shots--the maximum I use. I tried the Nikon D7200 ISO 51,200 and 102,400 and they were useful in getting shots that I couldn't get any other way. It looks like the E-M1 Mk II does a decent job at ISO 6400 but not enough that I would regularly use those photos. However, other people have told me with the original E-M5 that they went all the way to ISO 12,800 and the output was fine. YMMV.
As far as the Sony A7 Mk II goes, I see it like this: the A7 Mk II is
great for photographing flowers but the E-M1 Mk II will be great for
photographing flowers travelling on a lorry going 100 km/h.
Update 2016.11.19: I got to handle the E-M1 Mk II and 12-100mm f/4.0 lens today.
The lens was quite good--surprisingly good for an 8.3x superzoom.
Handling the E-M1 Mk II, I wasn't happy. Yes, it has a bigger battery but the grip is not great, for me anyway. The battery looked odd, as if it was pulled out of a dumpster and re-purposed. I saw some comment that the grip looked misplaced on a retro-styled camera body. I agree. They should have gone with an E-1 or E-5 style body but they would have to introduce a 3rd line of micro Four-Thirds camera bodies.
Usage didn't seem any different, but there is more customization--such as swapping the power switch and the multi-function lever.
The dual card slot was on the far edge of the rear of the camera behind a door similar to the one in the current body. It seemed an extreme position.
In fact, a lot of the E-M1 replacement looks as haphazard as the original--or any of the micro Four-Thirds Olympus models. I have no idea what they're thinking. I appreciate the internals though. I still like the E-1 dSLR best but it was technologically out-of-step even two years after it was introduced and the company took too long to introduce a new flagship.
Actually, they can take my money for a lens instead.
Update 2016.11.30: The reviews are looking fairly good, as expected. The nasty comments are also expected. People with other systems aren't happy that Olympus gets a lot of attention. They don't know anything of the 1970s when Olympus was bigger and better than Canon.
It's comforting to know that the C-AF mode can keep up. I wouldn't be using it since I had already modified my technique years ago when I started using the Panasonic GH3. Imaging Resource did a good job of working with the E-M1 Mk II, giving a little insight into actual use and the pitfalls.
Update 2017.02.13: Apparently, the E-M1 Mk II is very picky about UHS-II cards, not working with many of them. Then again, I see posts about some cards being recalled. There probably have been too many made with sub-standard components, in order to make extra profit. That doesn't excuse Olympus for their failure to be compatible, though.
I've seen a lot of comments of how the E-M1 Mk II doesn't work as well as it should. The production models don't seem to be as agreeable as the test models. I can understand that. Before the E-M1 was available, I was using some, with a group, and when I got mine, it seemed that things that had worked well on the pre-production versions weren't working so well on mine. Firmware updates have solved a lot.