This is on the MFT forum, so I'm assuming you're are talking about fly-by-wire manual focus on Olympus and Panasonic autofocus primes as opposed to manual focus using manual focus lenses. My experience is that manual focus with the Olympus primes is okay but I find noticeable binding on the Panasonic primes (14mm and 20mm). The binding problem with the Panasonic lenses gets much worse in cold weather. The binding requires extra fiddling to get focus right. My experience is that using manual focus on my PEN camera bodies with my Nikon-series AIS and Voigtlander lenses (via an adapter) is smoother by orders of magnitude over fly-by-wire.