So it's not based on the book, but based on the second movie based on the book. You still with me? 'I am Legend' is based on the movie 'Omega Man' and is superior to the Charlton Heston epic.
The film push's all the right buttons, great character development, good background, edge of the seat moments and is a solid movie. However it will not blow you away. Having read the book I went in with little expectation for this movie, I expected to hate it, I didn't.
A good, firm movie, although if you want to be blown away....buy the book.