Subjective camera preferences.....

I can definitely relate to this. I am previously a DSLR shooter with my Pentax K-5 then K-3. I bought a camera to mainly take photos of my family and kids but I would also occasionally do birding and wildlife when I'm trailing in the woods or hiking on the mountain. My main lens is the Tamron 17-50/2.8 and I bought a Sigma 150-500 OS for birding and wildlife. They both satisfy my needs very well in terms of optical quality. The Sigma 150-500 with the K-3 has the fastest AF among the lenses I have due to it's HSM feature. I am satisfied with it except one thing, it's just too heavy! It's extremely hard for me to bring this with me during trailing or hiking which is in itself pretty demanding in strength and stamina.

So I decided to try the m43 system and bought the EM5 + Panny 100-300, and I am immediately in love with the combo! The reach amount of reach I get with this small of a package amazes me every time I use them. I can fit my EM5 with grip and the lens in a small messenger bag such as the Lowepro 160 AW with ease and perhaps even able to fit in a 20/1.7 or 12-32 pancake lens in the front pocket! Now this combo is my go to gear whenever I go birding, it's just a lot easier. I still use my K-3 + 17-50 combo though for daily general purpose shooting.

I know some might argue that with cameras such as A7 and A7R entering the market, the weight and size advantage of the m43 is diminishing. They bring in the fact that an A7R with a 55/1.8 and/or 24-70/4 lens is not much heavier than say an EM1 or GH4 with 25/1.4 and/or 12-40/2.8 lens, which is basically only around 100g heavier than the EM1 both with the two lenses mentioned above respectively, and even no weight advantage for the GH4 combo. Hence they further argue that with a miniscule amount of weight difference, I get full frame for excellent low light performance and 36MP high resolution for details, it makes a lot of sense to go for the A7R combo. And I agree this is very true.

But what I think the m43 system is most advantageous on and that most people with these arguments above forget to mention is the weight advantage of m43 with telephoto lenses. You might be able to yield a comparable weight with the A7/A7R with normal focal length primes and zooms, but as you go further into the telephoto aspect, the m43 gets the advantage in terms of size and weight plainly due to physical nature, it's got a smaller sensor. The closest combo I can think of with the A7/A7r to the Panny 100-300, is perhaps a Tamron 150-600 VC + LA-EA4, and the weight of the 150-600 alone is already almost double the weight of the EM1 + Panny 100-300! Not to mention I can still go further and instead go with the GM1 + Olympus 75-300 combo for further less weight and size. Therefore, for me I use the m43 system mostly for telephoto shoots cause it's the area that's most advantageous for me. But of course it'll be a different story if I'm a pro shooting for a living, then I might go with a Canon or Nikon. Afterall, the m43 is not a "Let's shoot a black cat in the dark and count hairs" type of camera.

But for a hobbyist like me, the m43 is plenty enough for me to explore. The Nikon 1 system is close, but until they release the 70-300 dedicated for the Nikon 1 system I'm still going to stick with my EM5. At least I don't have to buy an FT1 and bigger APSC / FF Nikon lenses that are heavier and bulkier than my Panny 100-300.

In the end, it's purely subjective. Depends on what you need and what you value. I agree with the A7/A7R weight argument. Its a very tempting option if I shoot mostly in wide to normal focal lengths. The quality of the sensor with those G/Zeiss glasses are amazing. But they also come with a price, quite expensive actually. Hence, I'm going to stick with what I have and have fun out of it! After all, it's the photographer behind the camera that's more important than the gear.