Well, it links to past winners and their stories - do you rate the stories highly? If you are a short-story writer, have you come across the winners elsewhere? The quality of the judges is another point to consider.
I'm not a short-story writer, but the Bridport Prize awards are highly reputable, and include a short story prize with a £5000 first prize and various smaller ones. There are generally high-profile judges. Huge numbers of entrants, obviously, which does mean that to get read by the actual judges, you have to make it through lots of waves of first readers beforehand.