Originally posted by Vindemiatrix ...with all the problems I've heard of this lens...
I haven't been around here much lately, but I haven't heard anything about problems with this lens.
From my experience with the DA*55, there are only two circumstances I'll turn to another ~50mm lens: I prefer my Leica Summicron when most of the out-of-focus subject matter is in front of the focal plane, and I prefer my Auto-Takumar when I want blooming glare. Perhaps you refer to the SDM problems with the 16-50 & 50-135? Those don't apply to this lens, and incidentally the problem with those other lenses has been fixed for lenses manufactured this year. (And, my own pre-fix 16-50 has only had a few hiccups and performs perfectly so long as I use it at least weekly.)
For background blurring that I have not seen surpassed, even by cinema lenses (that cost tens of thousands of dollars), for crisp high contrast transition handling, and for all-weather knock-about use, the DA*55 is my choice and would continue to be my choice if I had an unlimited budget.
That said, from the above, you can gather that I've spent way too much money on ~50mm lenses. Based on this, I suggest that you use the DA50/1.8, find what its limitations are for you, and only if you find meaningful shortcomings to spend the additional money on a lens that meets those specific needs. I don't recommend buying the DA*55 because other ~50's are so good. The autofocus doesn't seem as fast as other lenses, and, when weather sealing matters, most people will turn to a zoom lens. But, because of how it excels compared to how I use it, it would be the last ~50mm lens that I'd part with and the first I'd acquire again when I had the means.
I think the allure of Pentax products is that they produce excellent but constrained products. Canon's competitor for the DA*55 is a 50/1.2, which is larger, heavier, and twice as expensive--and which, to my eye, sacrifices optical quality for that extra half-stop. Canon doesn't make a weather sealed 50/1.4 with superlative optics because that'd undercut the value of the 50/1.2. Canon wants you to climb their ladder. Pentax has no such worries: their market position seems to force them to design for functionality, and that has won my wallet for more years than I'd like to think of.