I really like my DA* 16-50 - it's most recent outing was my brother's wedding.
It's just a very solid lens - both in build and performance. Versus the kit-lens:
- ves
More chromatic abberation in general
Enormous
Built in flash vignettes at certain focal lengths with the hood on.
+ ves
Almost everything else - its sharper, more contrasty, silent AF, weathersealed
For me the weathe-sealing has been the biggest surprise. Surprising in that it's been more useful than I thought. These two shots would not have been possible without it:
Rain and spray off the sea:
Sand and dust: (although front element protected by filter)
Whether it's worth upgrading from the kit lens really depends upon what you want out of your images. If you want some of the highest image quality and are prepared to pay the premium it's worth it - as you already have the 50-135mm the 16-50 would be an excellent companion.