It's a quick read, but not a very good one. The existential angst is there, but because it's all conveyed through monologue, it loses all the subtlety of the film version. One of the movie's most brilliant moves is to have a series of encounters between Will Smith and a mannequin, which do far more to illustrate the misery of solitude than pages and pages of whining.
The book spends a lot of time on the protagonist's sexual desires, which is kind of intriguing (leading me to ask my boyfriend - hey! if you're the last man on Earth, with only crazy vampires and corpses left, which do you prefer - necrophilia, or raping a vampire? He said neither. Bo-ring.). It also goes into more explanations of the cause of the apocalypse, which aren't all that exciting or interesting. The ending is indeed totally different from the movie, but it's also kind of ... meh. I mean, I guess it's kind of a tired trope, but the real problem is that the writing isn't all that good. Not just at the conclusion, but throughout. Also, the plot is actually far less compelling than the one in the film, and does much less with the material in terms of raising interesting issues.
So yeah, overall, this is one of those cases where you're really better off watching the movie**.
*Incidentally, I'm led to believe that Hancock is somewhat similar in this regard. I'm curious.
** This is a small list. I'm trying to think of other examples - I definitely prefer The Princess Bride movie to the book, but there are some other cases too, I'm just blanking right now. I know there are two other film adaptations of I Am Legend, Last Man on Earth and Omega Man. I'm not really dying to see them. I've kind of lost interest in the story, for one, and for two, uh, those movies don't have Will Smith in them***.
*** In itself an interesting side point. My friend Russ and I were talking awhile back, and he was saying how he completely doesn't get why someone would watch a movie purely because a certain actor is in it. Well, I certainly do. Now, I haven't seen all of Will Smith's films, especially the more recent ones, which are apparently pretty terrible. But nonetheless - I'm definitely gonna see them all at some point. Because, well, I just like watching Will Smith do stuff. It's not just Will Smith. I also have a Tupac obsession and will generally watch pretty much any Ewan MacGregor movie as well.