He won't regret it. For all of the things that football fans say about the MLS, it is actually decently competitive, though perhaps lacking improvisation and flash, to a degree.
As for the Galaxy, they are a top class team which is, for some odd reason, playing like complete garbage this year. Luckily, unlike other countries, US leagues don't have any concept of "relegation", instead operating under a league-controlled "franchise" system, so he doesn't have to be worrying about dropping out of the top league.
When it comes to raising interest in football, however, I would have to say that, while he may still surprise us, there is little chance of him doing that single-handedly. Let me explain why I don't think so.
While we in America say that our national sport is baseball, the most popular sport is, with a fair degree of consistency, American football. However, I grew up in Chicago in the US during more or less the one time in recent history that that was not true.
When I was growing up, Basketball was, by far, the most popular sport in the US for one simple reason: Michael Jordan. Anyone who knows Jordan knows that he was famous not for his image or his off-court life, but solely for his game. Because he played for Chicago, I got to see him play live many times, and every time it was the same. He was consistently amazing, sometimes scoring more points than the rest of the team combined. He was so talented that, with Michael Jordan on the court, the game was literally all but decided. While basketball games now are usually decided by perhaps four points, kids when I grew up were used to seeing Chicago Bulls' games decided by point differences around twenty or more.
When Jordan stopped playing, you could feel basketball lose its glamor. It just wasn't the same anymore, not even close. I think that by the next season, basketball was no longer the most popular sport in the country, even though there were still plenty of talented and famous players in the association.
My point in telling this whole story is this: Americans are attracted not to famous athletes, but stunningly talented athletes. While Beckham is a big name and certainly has skill, he doesn't have the level of skill that could shift the entire country's attention over to a new sport. If MLS wants to capture mainstream America's devotion, then Beckham is not the player to bring in. The MLS needs to start bringing in extemely talented players who, like Ronaldinho or Robinho, are at their most entertaining ON the field, not off it.