Returning to shoot rugby this season, I had a bagful of new toys including a D4 body and my beloved Nikon 200-400mm f4 which went with me on the first shoot and I was very happy with the results. The combo worked perfectly (it always does), and yet, I still felt that even 400mm was not enough. Nor did I want to lose a stop of light with a teleconverter.
Accordingly, I do think 500mm is a great lens to shoot the rugby but as always, when you plump for a fixed focal length lens, you'll encounter other problems, most pertinently, when the action is close to you at which point 500mm is too much and then a second camera with say a 200mm lens on is what's needed.
where the lens falls down
Another problem was focus drift. The four shots below were a continuous burst such that no more than 1.5 seconds separated the four pictures. Picture 1 (top left) is in perfect focus for the player with the ball. In picture 2 (top right) taken a fraction of a second later, the lens for some reason switched focus to the trees behind such that the players are blurred. Picture 3 (bottom left) the focus is coming back to the players but given the short time frame between shots, focus has not yet been fully achieved so that the picture does not stand scrutiny when viewed at 100%. By picture 4 (bottom right) perfect focus is again achieved. I rarely if ever get this with a dedicated Nikon lens.
Most obviously then, at fast moving sports events, is getting 100% of the shots perfect (compared to say 75% with the Sigma) worth to you shelling out an additional £2k for? If you're a pro, probably yes, if you're rich, maybe, if your subjects don't normally move as fast as rugby players or don't move at all, probably no.
Sigma's 500mm f4.5 then really does fill a gap offering a quality lens that can deliver professional quality images. It is not 100% perfect, but if you want 100% perfect, that will cost you a further £2,090. For most people however for most occasions, the Sigma is likely to make you very happy.