I picked up one of these (Non HSM) recently. I can say f/4 is

**very **usable for subject isolation throughout the 100-300 focal range. I shot with it this weekend around 160mm and was able to do a decent job blowing out anything beyond 3-4 meters behind the subject. Someone who discounts the utility of a zoom usually hasn't used a good one

My impressions of this lens so far are very favorable. If you can pick one up (HSM or otherwise), I'd vote to do it. I haven't tried out the DA* 300mm, but I did own an A* 300mm f/2.8, and have my fair share of shorter DA/FA/D FA primes, so I'd like to think I'm fairly familiar with the Pentax telephoto "look." At these longer focal lengths, you set up with a tripod, and the zoom becomes

*pretty* useful real quick for composition.

The Sigma 100-300 f/4 so far is very much like an extended DA* 50-135 so far. PM me if you'd like a few examples.