The Last Man on Earth is by far the most faithful adaptation of the source material, apart from changing the main character's name to Morgan. It hits most of the key points and only really changes towards the end, but not too radically. It has only one actor, which is unfortunate as there are a number of people on screen. Also unfortunate is that it is very slow, which mixed with the bad acting, makes it a fairly ordinary effort.
The Omega Man very much has its own identity and take on things, and is only loosely based on the core idea of the book. However, it's dynamic, clever, and the 'creatures' are intelligent - they can talk and have a fanatical point of view - which only makes them far more interesting to watch.
I Am Legend - The new film is actually based on The Omega Man more than anything else. Unfortunately it misses one of the key points of the source material and the previous two films - a man alone in a world of monsters. The 'monsters' themselves are done using fairly ordinary CGI rather than actors, to the film's detriment. The film would loose nothing on DVD and is not really worth a cinema admission.
Of the three films, Omega Man is by far the best. It's the most interesting, engaging take, even if it only bears a light resemblance to the source material. Last Man suffers for it's pace and acting, Legend for its unrealistic looking creatures and scaring by surprise rather than mood.
I should be seeing I Am Omega tomorrow, all going well.