Will Smith is good in every thing he does. Problem though is that he's almost a stereotype just as himself. He cannot play a role without just being really cool and always kinda frivolously funny. An epic I Am Legend actually doesn't want that.

It's hard to say how good of an actor he is because he hasn't done anything with an awesome director (I think), and a huge part of how the audience perceives acting is from the standpoint of cinematography. Another issue is that I just don't think Smith fit the character. I believe the character in the book (which the movie should not have deviated from. Would have been sooooooo much better) was more a nerdy scientist, but Smith was a super cool scientist on steroids.

I think Ethan Hawke would have been the optimal choice had the movie been made more a drama and just all around better yet still looking for some of allure that Smith's had, but PSH probably woulda been optimal had the movie gone the route of pure epic, which I think it should have. PSH could knock that shit out the park. With an actor like him the movie would have to stray from the 'action hero' aspect and gone more into the human condition and such. Honestly, I think there is a severe lacking in apocalyptic dramas. They're all apocalyptic gores or horrors or thrillers, some with just a hint of what makes dramas so profound. I would absolutely love to see directors like Cameron, Spielberg, and Gibson tackle the apocalyptic genre.