Ha.. ha.. the real secret for getting the F*;
The Master (you know who you are

) here who has tried all the good lenses chose this one over all the *300mm

I've tried the DA* myself. Comparable sharpness at f4.5. Its a trade-off between slightly faster aperture, WR, SDM, a bit better tripod mount on the DA, vs the compactness/portability/handhold ease (and bragging rights) of the F*.
No wrong either direction.
Frankly, I find that the Sigma 100-300/4 to be one of THE BEST options for a good 300mm. Really sharp lens that rivals the *300 primes and matches well with a Sigma 1.4x teleconverter. Its a zoom too, so 100mm to 300mm is really well covered with just one lens. Downside is weight. The reason why I did not get it is that I don't trust Sigma (a few bad experiences) and I'd rather get Pentax stuff. I also have the option to borrow it if I want.
Yes, totally agree that the 55-300 is fantastic up to 190mm where it is only f4.5. On my limited comparison to a Sigma 70-200/2.8, I found that the Sigma at 200mm only matched in sharpness at f4.5 (ie. same) (so I did not buy the 70-200 in the end). Of course the 70-200/2.8 is a sharp enough at f2.8 and will deliver in low light situations that the 55-300 can't compete.