I prefer to use two bodies with a 28 and a 45-50mm prime lenses. When needed, I like to use a telephoto zoom (something of the 70-200 variety or close). In theory, I could very well use one body with a 50mm lens and another with a normal zoom too, but there are a number of reasons why I've yet to find a way to make it work for me in practice.
The first point is : why the 45-50mm prime ? That one is mandatory at all cost, and every other decision I make in a system is tied to that one. Well it's simply that I prefer the way this focal length looks and most of the shots I take that I don't dislike too much are made with that focal length, then 28mm, then in the telephoto range. Weirdly, I've never ever liked a shot I've taken with a 35mm focal length (but I do like a lot of shots taken with a 35mm focal length from other photographers), and I usually tend to crop a little shots made with a 40mm to something closer to the 45-50mm range. And I mostly dislike focal lengths wider than 28mm unless the focal length doesn't make itself known (either from me or others).
Now in the m43 world, the 12-40mm @f2.8 is pretty much on par with the Olympus 25mm @f2.8 in terms of resolution, but it's a lot bulkier and a ton heavier, is more than a stop slower (and with m43 sensors, every stop helps), and, in general, I've always had a psychological issue with zooms with wobbly inner tubes. What's the point in using something that's bigger, heavier, and slower if around 70% of the shots I'll keep will be around 50mm anyway ? It would be a needless operational hassle, especially as I carry the EM1 + 25mm combo with me every single day.
But I still bought it knowing that it was the best 28mm equivalent lens I could find for the m43 system (which highlights the lack of proper wide-angle primes in that system, or that, as far as 24mm is concerned, the 12mm f2 lens is a little too expensive for what it provides in terms of optical quality), that reselling it wouldn't cost me a dime thanks to a promotional event, and with the intention to try to use it with the second body on days dedicated to photography. But there its weight and size let me down, as I found it tedious to carry it on a second body for something that I'd mostly use at 28mm anyway. Also, I think it's flimsly made (I only used it once in the field and the black paint is already wearing off, and of course the inner tube wobbles and feels cheap). So I bought the Panasonic 14mm, which, relatively speaking, is pretty much a rubbish lens (it's a substantial downgrade from the Canon 28mm IS USM I used before, unlike the Olympus 25mm, which is an upgrade compared to Canon's prehistoric cheap and crapful 50mm lenses), but a lot smaller and lighter, and in line with the fact that I won't use it for more than 25% of my shots. I might buy the Panasonic 12-35 as a replacement, as it's smaller, and can't be worse than the Panasonic 14mm optically, or the 15mm.
Here we're touching another reason why I tend to prefer primes : manufacturing tolerances and quality control. At least that's what I though before trying my hand at some mirrorless systems, where I've witnessed insane levels of manufacturing tolerances and awful decentered rubbish thrown into the distribution system, regardless of if it's a prime or a zoom. For example, SLRgear's sample of the 35-100mm is clearly defective at 100mm. And Lenstip's sample of the 15mm is decentered (tilted focal plane). My first copy of the 12-40 was dismally decentered at 40mm (the second one is just fine though), and four out of seven Olympus 25mm I've had in my hands had a similar problem, a tilted focal plane on the left side, most likely a batch issue - and Photographyblog's copy has the same issue, easily visible on this shot :
http://img.photographyblog.com/revi...ages/olympus_m_zuiko_digital_25mm_f_18_36.jpg
So I'm not sure this last point is valid for the m43 system (and in my experience Fuji too).
