Will Smith was pretty good in this role, although his performance isn't really comparable to Tom Hanks' Castaway performance as some critics would have you believe. He definitely holds his own though. He pretty much carries 75% of this movie solo fairly successfully (and I think that says something). The role he plays is not your typical "hero" part - through the movie, it becomes pretty obvious that although he's a brilliant scientist and is doing one hell of a good job surviving on his own (well, with his dog Sam) he's definitely a little off his rocker. He has conversations with mannequins and even gets nervous when trying to decide whether to approach one of the female mannequins that he has positioned in the video store. All the while however, he's continuing his research into finding a cure for this problem - which is caused by a human engineered virus that was designed to cure cancer.
The visual effects were pretty cool. Seeing New York as an overgrown, abandoned, wasteland was pretty cool and the opening sequence of Smith hauling ass through the deserted streets of Manhattan in a Mustang while pursuing a herd of deer is pretty bad ass. The "dark seekers" as the monsters in this film come to be called are pretty freakin cool too. They're humans who have become infected with this virus that has altered them both physically and mentally so that they're now blood-thirsty, hyper-aggresive, pale, hairless creatures.
There were things in the movie that sort of seemed incongruous to me. Early on in the movie, Will Smith's character describes the creatures as having lost all human behavior but then later on in the movie, we see them very organized with a well-defined leader who are capable of setting elaborate traps and "domesticating" infected animals. Sort of strange, although it did move the plot along. Also, the science was a little sketchy at times, but I think this was pretty minor as well.
I kinda liked this movie. It's definitely worth seeing if you're a fan of genre films like this one and definitely worth seeing in the theater. The film give you everything you could want in a blockbuster movie - lots of action and great visuals. So, if you're a fan of the sci-fi genre, then you'll like this movie. It has an interesting aesthetic and a really good ending (I thought) and gives you a pretty good cinematic experience.