I discovered Scott Bourne’s photography relatively recently, he has a great podcast at www.photopodcasts.com. Scott quite publicly made a move from full frame DSLR to Olympus micro four thirds. As Scott is principally a bird photographer his system requirements are very different to mine, he makes it clear that he needed to change systems to reduce weight - that feels to me like his main driver for change. For sport and wildlife photography systems are always big and heavy with full frame, predominantly in this genre photographers are using very long fast lenses and there is no doubt the Olympus OM-D E-M1 Mark II excels in this area - great for long lenses, leading IBIS is very relevant. I have always tended to go wide, normally 20mm to 50mm equivalent - so weight is somewhat less of an issue, and stabilisation is less relevant for wide angles where longer shutter speeds are possible and use cases more allied with slower activities - travel, street where you need a somewhat faster speed to freeze motion - but there are of course some handling and cost advantages to a smaller, lighter system.

How much do I need full frame? It has measurable differences (I’m not calling them advantages).

• Around 5% of my pictures at 3200 or faster - without doubt there is a significant difference at high ISO when viewing images on screen, but less so in prints I have had made. 
  
• Around 33% of my pictures at f2.8 or faster - some is to manage light though I am happy to use high ISO on full frame, but a significant number of images is to manage DoF - this is a limitation with MFT but can be dealt with in part with the availability of faster lenses and careful management of subject to lens distance where possible. 
  
• A bit more latitude to crop - generally higher pixel count 
  

And FF has unmeasurable differences in terms of the nature of the files it produces. 

There is no doubt my E-M1 lags on IQ behind the Nikon D750, particularly with low light and wide angles. Ming Thein has an excellent article on format differences with which I agree. There seem to be subtle tonal transitions differences, micro contrast perhaps, maybe to do with bit depth but I am not sure. It certainly is very subtle. I think a part of this is down to lens characteristics too. I don’t think I can get a 35mm equivalent lens as good as my Zeiss Milvus 35mm f/2 on the MFT system currently, though the forthcoming 17mm f1.2 could be close.

Of course it’s not just the weight advantage, the E-M1 brings a quiet shutter, which is great for close street work and being mirrorless, the improved live view implementation is useful.

It doesn’t have to be about one system - there are lots of articles about Micro Four Thirds vs Full Frame but actually they are complementary not alternatives. FF will never be as small or light, Micro Four Thirds is getting close on dynamic range (and already exceeds film massively) but will never be able to compete on image quality at the extremes of sensitivity (ISO) when very large output is required. In film days, nobody would have seriously considered a Hasselblad medium format camera as an alternative to a Leica M6 or Nikon FM but many photographers used both.

APS-C now appeals less to me, too middle of the road, compromising in all areas, I am using my Leica X less - despite having a beautiful lens, I am now moving one way or the other, D750 or E-M1. If I liked Fuji and had it as a single system that would be fine, but I can’t get on with either the X-T2 or X-Pro 2.

At the moment I am slowly falling for the Olympus system, prints are surprisingly good and it is small and light with great ergonomics.