Like any sport, if you are serious enough, it can be a good fat burner and muscle builder (which includes proper training).
If you look at professional rugby players, they are all extremely well built. So people who play pick up rugby can also find them selves to be in great shape from the sport as well, as long as you have the time to dedicate in the gym as well as on the field. I think it tends to be like that in a lot of sports. Though I do suppose if you are looking to just burn fat mostly, a game like soccer would be better cuz it is a lot of cardio. However, Rugby can be a lot of cardio as well, even if you are a forward. Run to the rucks, be there to support your team mate every time and always attack on defense and you are running the entire game.
Like others have said, it is a brutal sport. Depending on the condition of your pitch (they are awful in the states and have been a bit better in Spain for me), you leave every practice and game with at minimum cuts over the knees and arms. Which are not fun to work out with. On top of it you are really sore, which is excuse enough to get out of going to the gym.
The best part about rugby is not the training though, it is the camaraderie you get on the team you play with.
Also...as for rules. The best youtube video I have found on rules/how the game works: http://www.youtube.com/watch?v=IEQyCcageGg