I guess I'll toss my opinion into the mix. Back when the first game first came out on N64, I imported the Japanese version, due to the fact that IGN was saying some good things about it, and the fact that n64 releases were few and far between, so I needed something to play.
I was a bit disappointed by the game. I am primarily a single-player guy, and the first game didn't have all that much of a single player experience. Every now and then I would get a 2 player match going with a friend, and it was pretty fun. I never thought it was terribly great, but fun nonetheless.
Then Melee came out for gamecube. This had a vastly improved single player mode, so this was the point at which I kinda started to really get into the series. The real kicker though, was on a few occasions when I got to play 4 player matches against various different people. It was suddenly a completely different experience! I couldn't believe how much fun I was having. I started learning a bit about the tournament scene, and I learned that the game is incredibly deep and that there are some seriously skilled players out there. Those 2 things really sort of changed my views about the game quite a bit, though it happened late in the gamecube's life cycle.
Now since Brawl was first announced, I have been seriously hyped for it, maybe unlike any other game thats come before. Once again, the single player game has been expanded SIGNIFICANTLY, and this time they are finally offering wifi play, so I can still get in some fun matches despite my lack of local friends
Another huge aspect of it for me is the nostalgic component of it. Smash Bros. Brawl is literally a fanboys wet dream. There is no other way to put it. I've been raised on Nintendo since the age of 5, and I'm now 25. Pretty much my entire life is filled with fond memories of Nintendo games, and Smash Bros. really brings back the feelings of the good ol' days.