Ever since the days of Karate Champ, there's been some sort of competitive scene for fighting games. That scene exploded with the release of Capcom's Street Fighter IV, and has been on a constant increase since the game hit U.S. shores in 2009. In addition, Super Smash Bros. has had a massive competitive scene since the Nintendo 64 days, and with the upcoming release of Smash Wii U and 3DS, it's unlikely that will change anytime soon.
It's not uncommon to see local Smash tournaments hit and exceed 50 people. Regional events can garner interest from hundreds of players, and Evolution, the biggest fighting game tournament in the world, hosted nearly 700 players for Super Smash Bros. Melee this year. With the huge sales figures of Smash games, it should be no surprise the franchise has good competitive numbers. However, you may be surprised with how the game plays at a competitive level. In most cases, items are turned off, only specific stages are legal and it's generally a one-on-one encounter with the occasional two-on-two team tournament.
One interesting fact about the Smash Brothers competitive scene is that it's split between Melee and Brawl. Nintendo made some changes in Brawl that didn't work well with competitive play (random trips), so while the community still supports Brawl, in almost every instance Melee gets a higher tournament attendance. Nintendo confirmed that Smash Wii U will be a happy median between Melee and Brawl, so it should be interesting to see how the competitive community receives the game. Most likely, Brawl will drop off the tournament radar, replaced by the new game, with Melee continuing as a side event at most Smash tournaments.
For online players, you won't find many tournaments to take part in. The online infrastructure of Wii U games isn't quite on the same level as PlayStation and Xbox online titles, so most competitive players shy away from online tournaments. You'll still find them in games like Street Fighter and Tekken, but for Smash you don't see them all that often. This will likely continue with Smash Wii U, so if you're looking for a tournament scene, you may have to leave your house to find one; most major cities have a large Smash competitive community, so you won't have to go far. Thankfully, website such as Smash Boards offer forums that help competitive players find each other and local tournaments.
The 3DS version of the new Smash Bros. game will probably not see much in the way of tournament play. If the games are close enough in terms of gameplay mechanics, you may see a lot of players warming up via the 3DS version while they wait for their turn to play at a tournament, but it's unlikely you'll find many 3DS-exclusive tournaments.
We all probably have a group of friends that love to get together and play some Smash. This won't change when the new games hit next year, but if you're looking to expand on that and take your skills to the next level, there will be plenty of tournaments going on for Super Smash Bros. Wii U. Sometimes it can be fun and fulfilling to take the next step toward competitive play.