When 4/3 came on the market with the first Olympus 4/3 SLR cams, I liked the idea, mainly because of the genuinely telecentric approach in lens design. What I did not really like was the 2x crop, because I mainly shot with adapted manual lenses in those times on my Canon EOS DSLR. The 2x crop "turned" any 50mm lens into a 100mm, any 35mm still into a 70mm tele and even a 24mm worked like a "normal" lens. It was almost impossible to shoot in wide angles when you shot manual lenses on 4/3. (The only way, before Samyang, were extremely expensive and heavy older 14mm lenses.)
So, I respected 4/3 but never went for it.
When Olympus came out with the first PEN as a µ4/3 system, I was kind of thrilled, because the camera looked fantastic, I loved the idea of using such a compact system that still offered a performance unmatched by any compact digital camera of that time.
I still did not invest in the system. APS-C had become the best compromise for me between size, performance, features and prize. First with Canon EOS DSLRs and later with Sony NEX system-cameras.
In order to try out and play with µ4/3, though, I later bought a used Panasonic GF3 for little money - and it basically it was just that for me, a toy.
After some time, I had the chance to start a cooperation with Olympus and was sent three of their cameras and several lenses for testing reasons and writing about them on my blog. Those cameras (OM 10, OM 1, PEN 5), really amazed me and most of the lenses as well. I was surprised by both the quality of the built and of the images. But since I already had more than enough systems (after a change, I shot with Nikon, Fuji and Leica), I did resist buying one of those cams.
Since some months ago, I every now and then shoot and write for Meyer Optik Görlitz. They send me lenses, I take photos and write short texts for their website. When they told me that I was about to receive a Nocturnus 0,95/35 for µ4/3, I quickly had to organise another camera, because I thought that the Lumix GF3 would not be an appropriate machine for that lens. On the other hand, I did not want to spend too much money on another µ4/3 cam. Fortunately, I found a mint Olympus PEN E-P3 for a really good price.
This cam then really convinced me (again). The µ4/3 system is a highly capable one that offers almost anything you might want for.
I do shoot with "fullframe" (Nikon Df), APS 1.3 (Leica M8), APS 1.5 (Fuji X-series, Nikon D7000...) and µ4/3 and in the same way that Megapixelnumbers have lost all their importance for me - "enough" is allright, I don't care if it is 10, 12, 16 or 20 - the sensor size is also secondary for me - "big enough" is mostly enough. (Again, only with vintage lenses, I prefer fullframe.)
I even have replaced my excellent Fuji X-T1 (APS) with the equally excellent Leica D-LUX 109 (µ4/3). Sometimes, portability is most important, esp. if it is combined with amazing performance.