THE OMEGA MAN is one of my all time favorite guilty pleasures. I've probably seen it a hundred times. It departs seriously from the theme and plot of Matheson's excellent novel, but they can both be enjoyed in their own right.
Probably the best adaptation of Matheson's book is an unauthorized one - THE NIGHT OF THE LIVING DEAD. George Romero has said he "borrowed" the theme of the rise of a new society (as well as some obvious plot elements) from Matheson.
WONDERFUL score for THE OMEGA MAN (I have it on CD, in a limited release) - Ron Grainer also did the score to THE PRISONER series and the theme to DR. WHO. Anthony Zerbe also gave a great over-the-top performance as Mathias.
Weird piece of trivia - Atlas Comics, a short-lived comics company from the 1970s formed by Stan Lee's brother, had plans to adapt a couple of 1970s properties to comics form, including THE OMEGA MAN. Charlton Heston actually did a written analysis of the film (which was reprinted in the Comics Journal), but the heads of the company decided that there was no need to purchase the rights to film and TV properties when they could just rip them off - thus, OMEGA MAN became PLANET OF VAMPIRES (no relation to the Mario Bava flick), and also contained some plot devices from another Heston film, PLANET OF THE APES:
...but which did have some cool Neil Adams cover art. It lasted 3 issues and was one of the few worthwhile Atlas titles.
They also changed their planned KOLCHAK: THE NIGHT STALKER adaptation into THE COUGAR.