i am sure you will have fun with your new 16-45. I enjoy mine very much and have little to say in addition to what i put in my lens review for this lens (26 of them here:
Pentax Lens Review Database - 16-45mm F4).
this is the most "contrasty" lens i have which i see as a very good thing. i do get vignetting on the 16mm wide shots, especially noticable on landscapes when the sky darkens up in the corners.
i never tried the *16-50 so i dont know how much better it gets, BUT, i have owned 2 copies of the DA 18-55 kit lens and this is a very obvious step up in IQ from those, no question.
I also used to own a very nice Sigma 18-50 f/2.8 EX and oddly i found the 18mm mark on this lens was slightly wider than the Sigma at 18mm and the 45mm mark on this lens zoomed closer to the subject than the 50mm mark on the Sigma...strange.