The HD DA20-40...
Firstly, for a zoom lens it's slightly limited in focal length range - never quite wide or long enough. So far as IQ is concerned, it's decent enough (good at the wide end, actually), but I've never agreed with those folks who consider it a "stack of primes" that can replace the DA21 f/3.2, DA35 f/2.8 and DA40 f/2.8 Limiteds. And the thing I have most trouble with is the border and corner performance at the long end. Our own PF review suggests that's due to coma, but I get the impression that the lens suffers from significant field curvature too.
It's beautifully built, feels great in use, and I know some people get great results from it. I really want to like it, but I've never clicked with it. That said, I've never disliked it enough to sell it either, and I keep telling myself that some time soon I'll spend a whole week shooting with it, until we've bonded