Melissa,
The weather sealing is indeed a big deal. And it works. The only weather-sealed lens that we have is the 100mm (macro) and it's pretty amazing (tho the bokeh doesn't compare to the Tamron 90mm macro - but it lacks the weather sealing).
If I were going to get only one lens for the K-3 (and weather sealing were an issue), I'd get the 18-135mm WR. It's comparatively quick and people have been getting some pretty astonishing results from it - even fairly close:
http://www.pentaxforums.com/fo...
It's obviously not a prime, but there's something to be said for having a capable zoom, too.
