I gotta say that I'm impressed by Beckham's first game. Not by his performance obviously, since he only played about 15 minutes due to injury, but due to the crowd he was able to bring.
The stadium was sold out for 27,000 people and he brought an a-list of celebrities to the game: Arnold, Eva Longoria, Kevin Garnett, Katie Holmes, etc. etc.
Those are more the statistics of an all-star basketball game than an exhibition soccer game.
It's obvious Beckham will be the star of the soccer world marketing wise, but how will be perform as a player? Personally I think he'll do very well since he handled himself playing against the best in Europe. Personally his opponents in the MLS are not in his league and will make him look better than he actually is.
Most likely he'll have a great 5 year run. After that I don't think he has much to prove in soccer and will probably move on and venture into Hollywood.