First of all, do not read the synopsis! Go into the movie with a clear mind and be clueless about the plot of the movie. (Especially the synopsis on this page...just don't do it! Look away!)
This movie is like a puzzle, with a lot of flash backs and scenes that don't make senes right away. However, as you progess through the movie, you discover who Will Smith's character really his - his history and past, and what he plans to do with his future.
It may be easy to guess what the climax of the movie will be fairly early on, but if you don't there is one critical part when everything will click.
The movie has a bittersweet ending, and be ready for some tears! However, it was absolutely excellent.
Side Note - I went to the premiere for Seven Pounds in London, and Will Smith announced to the crowed that he hoped we all enjoyed the movie, as it is very different from other movies he has done. I believe he was right and he did a fantastic job. His acting was flawless.