Originally posted by fwcetus I agree with northcoastgreg 100%. I use a K-3, and an F*300/4.5, and now also a 55-300 PLM (while I used to use a 55-300 pre-PLM along with the F* 300/4.5). I use the F* when I can and the PLM when I can't (or when I need a compact walkabout tele), and I am (and have been) quite happy with my 300mm coverage (and would be nearly as happy with the DA* 300/4 or the FA* 300/4.5. along with the 55-300 PLM).
I also agree with Greg. My experience is similar to @fwcetus. I have an FA*300 f4.5 and it's a gem. But for everyday use, and walking around, the 55-300 is very useful, and its performance is more than adequate. In good light, and at f8, with a subject not too distant, the zoom can give very high quality images.
Where you really see the difference between a consumer lens and those a level or two up is in testing conditions - more distant subject, lower light, wider aperture, etc. But that extra performance comes at an often-hefty price (and usually a lot more weight and bulk too). For many users, a good consumer zoom is all they need. But, as Greg said, if you do end up getting a premium long lens (like the DA*300 or the DFA 150-450), the consumer zoom will still have a place in your bag.
I also agree with the earlier comments about MF. I used MF lenses on my film SLR for many years and got reasonably good at MF. But once I went to AF lenses - even a screw-drive superzoom on a K100D Super - I got far more keepers with non-static subjects. All power to those who master MF for wildlife, but AF makes things a lot easier.
And the MFD point mentioned above is another reason to prefer the more modern AF lenses over the M or A series tele primes, or lenses like the Tair. The newer lenses also tend to have better coatings and better correction for CA. And for nature photography, WR is also a significant advantage.
As between the DA 55-300 models, I have the DA-L and the PLM versions. There is not a lot of optical difference between the screw-driven versions (which are identical to one another, except for newer coatings on the WR version) and the PLM version. At f8 you could scarcely tell the difference. The screw-driven versions are not internal focus, and so don't suffer from focus-breathing, so you get (some) more magnification of subjects which are not too distant. Although the PLM is one-third of a stop slower at the long end, my subjective impression is that it is just a little better at f6.3 than the DA-L (at least the bokeh is better), and if you want f5.6 you can back off to about 280mm. The major advantages of the PLM are its quiet and fast AF (the noisy AF is the major drawback of the screw-drive versions), its shorter MFD, and its compact size when retracted.