The movie is very good, especially given that it relies on a single actor for the vast majority of the time. Smith does a great job playing a guy who is clearly going crazy from being by himself for three years yet is brilliant in his moments of lucidity when he works on a cure for the virus. It is one of the better acting jobs he has done in his career. I like the alternate version because it fleshes out the story of whether the vampires are becoming sentient beings vs just mindless killing machines. It really goes deeper into the ethical dilemma of whether it is right to "cure" them. It is a question that the film does not really answer in either version, but it does make you think. As of this writing, there is talk of a sequel being made based on the alternate ending, so it is worth the time to watch the alternate version of the movie.
Apparently, the movie diverges a lot from the source books, which upset a lot of fans. If that is something that would take away from your enjoyment of the movie, then you should probably skip this. If you are a fan of Will Smith, the sci-fi kind of dramas and stories (especially ones that do make you think some), and can let the movie stand on its own, you will probably find this enjoyable. My only real gripe with the film is the fact that they made the vampires CGI. I do not think that there was a big need to do so, and the film could have been even more effective if you put real actors in that role.