I use a Pentax K10D with the big Sigma 50 - 500 mm lens. This lens is very sharp at F8 . However when you want to use it for Bird in Flight Shots it is a slow lens. I have been in the market for a faster lens for B.I.F.S and therefore looked at the above prime lens. I was lucky to borrow this lens from a mate of mine to try out. I used the lens with my K10D. I found the lens to be sharp from F4 onwards. I found the lens sweet spot at F5.3 where I really got some great shots. With my K10D the auto focus was not as fast as I would have liked for B.I.F.S , however I still got some sharp shots in flight. I am also looking at buying the Pentax K5. With this camera the auto focus will be fast. The only bad thing about the lens is that there is not really a good converter for it. The 300mm is fantastic for near shots.
Below are some shots that I took with this lens. Make your own conclusions.
Please let me know whether you think I should buy this.
PS. Your comments might help me convince my wife.