Along with primes I shoot f/2.8 zooms so the weight of the FA31 isn't really noticeable to me. I guess everything is relative. I haven't shot other primes in that focal length so can't make comparisons based on personal experience but from all the side-by-sides I've seen done online, I always prefer the FA31's rendering when it comes to the out of focus areas. I often like to shoot wide open, or close to, and the majority of my shooting ranges from wide to medium telephoto. On APS-C the FA31 comfortably falls right around a normal FL so it's super useful and multipurpose for my style of shooting. The FA31 has been on my camera more than any other lens ever since I got it. In truth, I only have a couple of quibbles about the lens. While I like the appearance and general functionality of the built-in hood, it complicates using a filter that's larger than the 58mm diameter. Sometimes I wish it had quickshift like my DA lenses have. Lastly, I'll agree that the size is a touch on the big side, but other than how this can be slightly impractical at times I actually like the substantial feel and appearance it gives the lens. The design of the FA31 and the FA77 pair so well with the K-5 I find them to be the sexiest of any DSLR and lens combo out there.
My FA77 gets a lot less use (Edit: meant to say does NOT get a lot of use), due to the longer focal length. I hope to change that though and adapt to using it more. I've found that the FA77 can have more purple fringing, which I'm not crazy about. Also, as it turns out, my FA77 is the only lens I own that requires any focus adjustment.
I would follow Chaos-Realm's advice of carefully evaluating the pros and cons. If you consider the alternatives, depending on what's most important to you, you might discover that the FA31 is the best lens for you, or you might not. I think the DA35 f/2.8 Ltd macro would be too slow for me, both in terms of aperture and focusing. But its quality build and quickshift would make it a joy to use. The DA 35 f/2.4 would be too plastic fantastic for my tastes and I don't think I'd be satisfied with the bokeh, though the focusing is nice and fast. The FA35 f/2, I don't know, maybe that would be a good option and it'd let you stay FF compatible. Though from what I understand, the DA35 f/2.4 is also FF compatible. Maybe Sigma has some lenses you could consider, but I don't think they're going to save you much in terms of size or weight.
Best of luck!
Last edited by TomTextura; 08-30-2012 at 12:07 PM.