Originally posted by moshepupkin Why would I want a manual lens for general purpose photography?
You will always find MF fans in the Pentax community. And indeed these lenses can be good and inexpensive, but it's clearly not what you're looking for.
For the record, I believe you're refering to f2.8 lenses?
I'm not completely surprised you get AF problems with your Tamron 17-50, I have to colleagues (using the Nikon version) who have had problems too. The lenses were fixed under warranty, and they did not "die". I would start by contacting Tamron about this.
There is a review on this site comparing the Pentax 16-50 to the Sigma and Tamron equivalents. You should consider reading it.
The only other alternative I see is a Sigma 17-70 f2,8/f4. It's a bit slower at the tele end, but it has a longer reach and better close focusing capabilities. Many people (including me, even though I own the older version) love the versatility of that lens.