I'm the proud owner of a Pentax K20D, which is also my first DSLR (and SLR in general).
Took up photography this past summer (summer 2009), but I've put a lot of time and research into it so far. But I'm far from a professional, of course.
Pentax 16-45mm f/4.0 SMC PDA ED AL Zoom Lens for Pentax and Samsung Digital SLR Cameras which, by the way, I have been pleasantly happy with so far as my standard lens. It's also great for me personally, because of my landscape/panorama interest.
This lens performs its duty well. I'll break the review down into further sections:
Auto Focus Feature: It focuses fairly quickly, smoothly, and accurately in decent light, and hunts around for a little bit in low light settings (took it into my windowless bathroom and left a crack in the door), but not as much as other consumer (price-wise) telephoto lenses.
Auto Focus Sound: To me it's a pretty soft sound/not too loud, but you will notice it.
Optical Stabilization (OS): Works amazingly and wonderfully, and it can stack with the anti-shake feature built-in to the Pentax body. Slight caveat though-- it makes this high pitched humming sound when it's working, maybe not audible to older ears (no offense here), and definitely not audible in louder areas such as sports events, but when you're using it quietly indoors-- you might have a slight problem with those sitting next to you. You do get used to it after a bit though, and it stops as soon as you stop using the OS feature (finish the shot/stop pressing the shutter half-way).
Weight and Dimensions: It's fairly big (I believe the dimensions of 3 x 5 x 5 in the description is severely understated), but smaller than other telephoto lenses from what I understand. It's got a bit of heft to it too, but not back-breaking or anything; it IS significant though, so keep that in mind if you plan on bringing it on a long hike or something (maybe walk around with it + your camera gear in your camera bag for an hour or so just around the house to see if it's manageable).
Functionality and Performance: It does what it's supposed to, and quite good at it too. As usual, with full telephoto range (300mm), shaking and stabilization become a big factor and issue; the optical stabilization helps a lot, as does the built-in Pentax anti-shake, but you're still definitely going to want to bring a tripod if you have shaky hands, or want very clean pictures. This is an issue with most if not all telephoto lenses though. The zoom on it is nice, and make sure you do pay attention to the fact that it has a minimum focusing range. The fact that it's f/4.0-5.6 instead of f/4.5-5.6 means a little quicker shots, which is definitely a plus over the competitors. Update: So, low lighting/indoor situations, as predicted, are not too great with telephoto lenses; the low shutter speed you have to sacrifice for, if you're going into telephoto ranges, makes it impossible to get a clean shot (without blur) sometimes; have something to brace against, or as I said before, a tripod. I'll be updating later on nature + outdoor shots.
Quality of Shots: The quality of shots is extremely nice; I haven't noticed any vignetting or aberration in the photos, but so far they're been indoor + medium-lighting shots. Blur is an issue with telephoto ranges, as mentioned before, because image stabilization can be only so good. More updates to come.
Ease of Use and Installation: It's very easy to install, just line up the focus line with the arrow and then turn it until the click-- it's very snug and fits well. To use the auto focus mode, switch the tab from (M) to (AF), and do the same on your camera (from (MF) to (AF.C) [confirmed to work] or (AF.S)). Optical Stabilization (OS) is also a tab, and you can switch it between On and Off; if it is On, it uses the camera's battery and will drain battery faster than normal-- so turn it off if you do not need it. Focusing ring and zoom ring are standard fare; although, if you have the lens hood on (or filters), beware-- if you point the camera/lens straight down, the focus ring may shift and the front of the lens "pulled" out by gravity. So far, it seems to only be happening sometimes, so I think it's a common occurrence; I could be wrong and just have gotten a defective one (kind of a big pet peeve for me, so it loses a star in "Quality of product" for that).
If you are looking for a fairly good, well-priced, third party telephoto lens (for your Pentax, or they make them for other brands as well), then you may very well want to consider this. With premium optical stabilization capabilities, the quality of which is usually only found in higher-end models, and auto-focus that works with the body, this is definitely a telephoto lens you should not miss out on. If you are a professional or are aiming to utilize close-up shots/telephoto lenses a lot more often as your default lens, then you might want to wait to just get a prosumer/pro one, but for me as just a secondary lens, and also on a tighter budget of a consumer, this was an excellent deal for me.