I've used m43 (E-M5) and liked it a lot, except for low-light AF and high ISO performance. Also preferred optical viewfinder to EVF - I don't know how the EVF on current top end stacks up (E-M1, X-T1, A7/r). My hands/wrists definitely appreciated the weight savings. New sensors are closing the gap nicely. Difference in depth of field is something to consider for some folks, though, for m43.
I thought long and hard about trying out Fuji system but its AF wasn't snappy enough for me, at least at the time (kid moving unpredictably). I'm happily back using DSLR.