The comic in question is pretty much just the source of a few names and the first action sequence at Bruce Willis' house (Warren Ellis, the author, said this is fine; the original story would, he said, have been good for about half an hour of movie), but it's a good movie nonetheless.
Mainly, this is a film where you come to see the stars have fun within their standard personas. It's not overly concerned with the plot, though it does have a few small bits of playing with expectations that I wasn't expecting. Tonally, I think it veers between serious and unserious in a manner that may not completely work (Morgan Freeman getting killed midway through seems out of step with everything else, though it does supply some actual jeopardy to the remainder of the movie; you can almost buy that Helen Mirren is done for at one point).
Also regarding Freeman, it's an interesting case of how advertising can shape your view of a movie - he's all over the trailers, but Bryan Cox (with a fairly un-hammy Russian accent), who basically takes his place midway through, is nowhere to be seen.
The cast is great, as you'd expect. Willis is still a plausible action hero - actually, this is almost a bit of a problem, since everybody else is plausibly old, but Willis (the youngest to begin with) doesn't really look much different than he did 15 years ago. My favourite parts of the movie where the scenes (particularly the early ones) between Willis and Mary-Louise Parker (the reaction when he shows up at her apartment, the subsequent "adventure").