Sorry if this was posted allready; as I can't find it if it was, so...
SMC Pentax-DA 55-300mm f/4-5.8 ED Lens Review
This backs up what I and others have been preaching and moreover showing for over 3 years now!
(from my post on DPR)
This backs Photozones review but is a bit more in depth in numbers and subjectively. Klaus said the lens shines in the field with snappy contrast. This review backs that finding. I have not seen many lens reviews using the words strong contrast as much as with the Pentax DA55-300mm ED. The reviewer has some nice pics that shows and explains how its snappy contrast and good Pentax color go a long way toward making this lens just really good.
Its a lens that is good enough to keep 'many' enthusiasts happy. It has great to good IQ from 55-300mm; and wide open(!), controlled CA/PF that's not intrusive, very contrasty (probably did help me in the BIF shooting/tracking), great Pentax colors (I believe its a real thing), small and light to carry because it was
not made for FF! That's the key for its price to performance ratio. I believe a lens optimized for one sensor is better than one used on both APS and FF; you lose value or some other trade off to do so.
This lens offers enough IQ at telephoto (take a look at the reviews pics, or pics here at DPR) all the way to 300mm wide open that many just can't justify paying $1000 more for a FF lens. Yes it will be better. But darn are they close when it comes time to look at the pics, close enough many a time, sure not everytime. But the Pentax DA55-300mm does not make a bad pic at any setting if everything else is right the pic will be good.