I know very little about other competitive and multi-player communities. All I know for certain is that the game has sold millions of copies which means you'll find people all over who play it. Its not like Vanguard Princess or some other random fighter that people have never heard of before. So, the community for smash is a lot larger than one might actually think, and it is active enough that it holds regular and yearly national and international events. Although, most of it is region specific, but you might always be able to start up a local scene. With some effort, you could push to have tournaments by talking to local stores or stuff.
I believe people probably find it odd that there is a competitive community for smash. Despite it being so well-played, I always hear of stories of people who got in thinking, "there's a competitive scene for this game?" Smash is probably one of the better underground communities. I call say that since Smash gets virtually no support from any sort of big time backing. MLG is the only group to have ever sponsored large scale tournaments. Almost everything else such as the nationals have been completely driven by great tournament organizers who put in a lot of time and effort.
In the end, its hard to nutshell an entire community. I have been speaking very broadly since Brawl, Melee, and Smash64 each have their own followings within a much bigger smash community. Some melee players and brawl players will bash the other game for design or joke about it, while plenty of other people put in time to get good at all the games. I have no sense of the smash64 group at all. From what I can tell, most of this site is driven by melee/brawl.
The one piece of advise I can offer is smash is pretty deep and its all about the amount of effort you want to put in and maybe what you want to focus in. There are some really talented players with years of experience in melee or brawl or smash64. If you want to get into competitive play and seek the international or national or regional levels, you'll have to put in a ton of time. I'm talking you could probably play for a couple years and you will still find people who will 3 or 4 stock you. If you do not care about that all that much, then its still a ton of fun with a local community if you stick with it.
Oh, and there are a few sub-aspects too within the smash community. Like hacking with Brawl has come super far since the game first game out, and you can find people dedicated to whipping out new character animations and textures. Even imports from other nintendo games are possible. Again, there is a lot to offer with smash and it all depends upon how much you want to get involved.
