I got into M43 to bring down the weight of my travel gear. I have a Panasonic GX7 and a GX8 and several lenses. I kept one of my DSLRs (D750) and my birding lenses (Tamron 150 - 600 and Nikon 300mm f2.8) because I heard that the AF on M43 system was too slow. So I have only solved half of my weight problem as wildlife shoot to Africa or elsewhere will still find me travelling with heavy gear. I am really hoping that the coming Panasonic 100-400 f4-6.3 will work as well for me as it seems to be for the guys testing it in this video.
http://naturalexposures.com/leica-lumix-100-400mm/

Watch the seven minute video.

Yes. Zero regrets. LUMIX GH4. Do it!

I do a blend of still and video production for training, portraits, and more. The GH4 makes all of it easy. Travel is a breeze with a body and two pro zooms (12-35 and 35-100). The new 100-400 Leica zoom is on my wish list, along with an updated 7-14mm. These four are the equivalent of 14 to 800mm on full frame! A few select fast primes (15mm, 30mm macro, 42.5, and 75mm) add very low light capability and great bokeh and subject isolation. Any equivalent coverage I have is about 1/3 to 1/4 the size, bulk, and weight of full frame.

If I were a working pro sports photographer, I'd use full frame. If I sold wall-size landscapes or large point-of-purchase posters for store windows or ceilings, I'd use full frame.

Best advice if you want to switch --- Do your homework. Olympus, Panasonic, Sony, and Fujifilm make the best mirrorless systems, and they each have distinct advantages and drawbacks. Be sure you handle them before buying! Hand feel varies a lot from body to body, person to person.

Nikon's 1 system is for amateurs, not enthusiasts and semi pros. Canon's M series seems to me to be a mistake. I know folks who have them and like them, but they are not advanced photographers. It pains me to say that, because I've used Canons and Nikons since 1968 and like both for different reasons.

I switched, but without as much cost as most of you would experience. I recently started with DSLR equipment including the Canon t3i, Canon 18-200mm and Canon prime 50mm f/1.8.

I needed new equipment, anyway, so I took the plunge with a new Sony a6000 with a kit lens. I'm still amazed at what that camera can do and on my second trip over to the store I bought a Sony a7RII, Sony 70-200mm lens and the Sony 28-70mm lens.

As impressed as i am with both my Sony cameras, they don't have affordable lenses with enough reach for action or wildlife photography and Sony does not recommend using the doubler and even so, it's not available yet.

They just announced 3 new Master G lenses including the 70-200mm and the 24mm-70mm. Therse are a f/2.8 version of what I already bought, but these lenses can use doublers.

I don't know if it's just me learning how to work these very techie cameras or if i bought too soon. I am having focusing problems when it comes to tracking fast moving subjects. If i continue to have these problems, I intend to buy a Canon 7D II w/Canon 400 prime to use when i shoot surfers, birds, dogs at play or anything that moves quickly and need good auto focus and tracking.

I haven't exactly switched - still have full size Nikon DSLR. But I choked at the price of an underwater housing for that, and found a bargain on a Nikon 1 S2 with housing. I now have two Nikon 1 cameras, an S2 and a J4, because I always want to have a backup. Image quality appears very good, not equal to a FF camera, but good enough for any printing I will do (limited to no bigger than 13x9) and way more than needed for screen display. Raw files are easy to work with in post processing, and when I'm packing for a trip two cameras with lenses fit in the same volume required by one 55-300 lens. I don't like the viewfinderless framing, and miss being able to see the depth of field limits in my viewfinder, but for most picture taking the mirrorless cameras work pretty well.