[jatrax]

I use a K-3 for birding. The DA*300 locks more quickly and will stay on target better.

I would agree with this, for BIF the DA*300 is noticeably better. The 55-300 will quite often lock faster on stationary objects but with BIF if it misses it will rack out and back and by that time the bird is long gone.

[northcoastgreg]

Sdm does better tracking, because screw drive has less precision in small micro movements. Gear drive always has some backlash.

That seems to be the main advantage of SDM. And it's not merely a matter of tracking; it also helps accuracy. My DA* 300 is my most accurate focusing lens, despite the fact it's the lens where focus accuracy is most critical (because of narrow DOF). My screw-drive Tamron 70-200 seems to focus faster (although that might just because it's noisier), but it does not focus as accurately.

[robthebloke]

I just took the dive on a black friday DA* 300mm. Wow! The autofocus is fast (faster than any of my screwdrive lenses, and much much faster than my 50-135). It just seems to lock on, and boom!
I've never used the FA* or A* variants so can't comment as to whether it's better, but the DA* 300 has certainly impressed me enormously.

[audiobomber]

I've never used the FA* or A* variants so can't comment as to whether it's better, but the DA* 300 has certainly impressed me enormously.

Not much to compare with the A*300mm f4. Even ignoring AF vs. MF, the DA*300 is sharper, with no CA vs. significant CA on the A*.

[RahZee]

I've never tried any of the F lenses, but I've recently got the DA* 300/4 and of course I've put it to the BIF test right away with my K-3 II, just out of curiosity since that seems to be the preferred focus speed test for tele's.
I'm not gonna start a fire here by comparing this to other similar lenses+cameras combos from other brands, but since this is my first Pentax camera I do have more terms of comparison and probably reasons to put on a smile...or not.
The DA* is definitely on the slow side of focus performance, though surprisingly silent. My first shooting session made me think it is much better suited for wildlife, where silence is crucial....IF the camera would only have a more silent shutter too.
Surprisingly for some, I have not purchased this lens for BIF, but for portraiture, a particular type, since I shoot a lot of variety shows. And for that purpose, the lens is actually better than what the camera can deliver. I'm talking chaotic lights (dazzles the Auto WB), high ISOs, high fps and shutter speeds starting from 1/300. Add a very limited buffer and a slow writing speed on the card and you get (well, I do) that feeling that you were better off with one of the big names.
Still, apart from the slow focusing speed and maybe it's size and weight, there's not much to complain about this lens. It does a great job and there's only that much you can ask when the flagship APSC camera can't keep up with it.