Originally posted by GibbyTheMole As it happens, I have both the Pentax DA-L 55-300 and the Tamron 23A SP 60-300. I've A/B compared the two pretty extensively. The Tamron is an excellent, very sharp lens. But from about 80-300, they're very close to each other in terms of IQ. (The Tamron is sharper below 80mm than the Pentax.) I'd say sharpness is about even between the two from 80-300, and the Pentax has less CA. Not to mention unless you have the spendy PK/A adapter, the Tamron is totally manual everything. One edge the Tamron has is the excellent true macro capability.
Basically, they're both keepers. :-) I use the Pentax 55-300 more, though.
PZ agrees that the DA55-300 is a decent lens. I retract my supposition.