Man was this a truly great read,there were so many awesome moments, Logan using the Iron Man suit, and killing the President with Captain America's shield. 
 
This is the first of Mark Millar's work that I've read, does anyone have suggestions of what to check out in the same vein as this?

Read Fantastic Four #554-569 and the miniseries 1985...both are written by Millar and all 3 sort of work as a trilogy of books

Ultimates.

Civil War. This Millar-comic brought me back into comics, it's pretty awesome as well (and the creative team is the same as in Old Man Logan).

I checked out Nemesis, I liked it a lot actually