Apparently they didn't think any audience was intelligent enough for the plot of the book, which was actually quite deep.  Instead they gave us some cheap crappy thriller designed for nothing but the brainless action movie fans.  

Can someone explain to me this alternate ending ?My internet conection is slow right now (I am at my parents house ) and cant wait until that thing loads ..

By the way ,I did read the book ...

Movie was OK except for the horrible ending ...

I understand that he says he is Legend because he is no more ....he takes something to die that Ana/Ruth gives him before the vampires kill him with pain ,and in the final reflexions of his life he realizes the world now belong to the vampires ,they are the only ones left and will create a new society that he may not like but its the same ...he was the last man in earth and now that he dies he is nothing but a legend ..

Everybody who has seen the film should read the book.
Also watch this:
http://www.archive.org/details/the-last-man-on-earth

Ok I watched it, but I haven't seen the movie yet. Is that bad?

I liked it just as much as the other one. What happened, though? Did he not cure the disease or something?

Have to disagree with you on this. The movie was awesome until the zombies started attacking his house. This ending was definitly better than the original but it definitly could have been better.  But everyone is entitled to their own opinion. 

I'm going to go against a lot of people here and say that I prefer the original ending shown in the cinema (which still wasn't the greatest). This alternate ending is too anti-climactic, and predicatable.

will watch this later, as i strongly disliked the original ending, and was hoping for something that concludes the movie, if only a little, better

No.  Absolutely not.  It's is definitely one of the finest movies to come out in a long time.  It successfully blends action, suspense, and horror with human emotion, psychology, and an intense sense of isolation.  That said, why would you watch the ending to a movie you haven't seen yet?

This ending is much better and actually makes sense when compared to the rest of the movie.