Just read Absolute Promethea Vol 1 and enjoyed it. Never got much into America's Best Comics, so reading Promethea's millennium overtones now only testified to it having stood the test of time. Having JH Williams III's art big really makes sense, so this makes this Absolute edition one of the better ones to get. Some of you would know that I only got into JH Williams III recently via Batwoman and was totally blown away by it.
(excellent analysis of Williams' art in Batwoman:
Only thing missing are the Moore scripts (other Absolutes had them) , but maybe that would appear in Vol 2.
In the meantime, try to find a copy of the Promethea Covers Special (2005). All the covers with notes by Williams. Now that should be reprinted in the Absolute format.