The DA 17-70 certainly does not "suck" over 50mm. If you want to shoot >50mm, it's certainly a better better choice than DA*16-50, Sigma 18-50, Tamron 17-50, etc. [none of which offer FL >50mm
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I did a comparison at 70mm against many other lenses, and my general sense puts it at #3 behind the DA70/2.4 and FA24-90/3.5-4.5. (Not surprisingly, the DA70/2.4 really outclassed everything else I had available. The only lens I wish I had to add to the comparison would have been the DA* 50-135.) I have always had somewhat mixed feelings about my 16-45--which were generally addressed by the DA17-70. In terms of build, it's considerably better-feeling than the DA16-45. Having the 45-70 range is important to me--so much so that I've been trying lots of F and FA film zooms. But this nearly always meant bringing another lens for the wide angle--sometimes you just want to bring one lens. I never seriously considered the Sigma 17-70, partially because I already owned the 16-45 and some film zooms, and also because I don't like that Sigma's zoom & focus rings operate in the opposite direction from Pentax's, and I didn't really want to deal with the Sigma's 72mm filter size.
I have not been terribly impressed with the short end of telezooms like DA55-300 or DA50-200 for this purpose in particular because the minimum focus distance is often an issue for me, and their use is likely to result in a good deal more lens changes for me. I don't necessarily need a macro lens but I like to be able to focus fairly close for headshots--and the long ends of standard zooms usually focus closer than the short ends of telezooms.