Joining this a few months late, but I have been doing crossfit on and off for 2 years. Firstly I just want to say it is the most fun I have had working out ever. Secondly I think it works.
If you are going to do it for Rugby, I would not suggest just simply following the main site WODs, as that will be far to general. Use your head and find work outs that are more relevant to what us rugby players do. I have found that I will move slightly away from things that are strictly "crossfit" into things that are better described as circuits. But does that matter? No.
At the end of the day, rugby requires such varied conditioning, so doing any exercise program that varies what you are working on is a good place to start.
Crossfit has got me doing things like burpees, good explosive power from the floor. Plyometric jumps, great for lineouts. Running and weights one after the other with no break......ideal for rugby.
Could another program have got me doing this? Yes.
Crossfit isn't going to make you an expert at everything. But it has some great workouts. Don't be afraid or put off it.
My biggest dislike is the cost of membership. I have never been inside a crossfit gym, I have my own weights at home, and I decide what workouts I am going to do so that the training is all relevant.