Some spoilers ahead.
I'm a huge fan of Angel, but I was shy of reading the season 6 comics for a while because I liked the way the series ended and I wasn't much of a graphic novel fan at the time.
It turns out I didn't miss much. I declared this book to be un-canon... not because it was bad stuff, but because it was just so boring and unsatisfying. The things I didn't like about season 5 were still there to be disliked, and while the book was very intense in places, it was all about all the awful things that had happened to the characters since we'd seen them last. Which would be fine, but... nothing terrible really happened. At all. And what things did happen were the things that happened onscreen in the series, there was nothing NEW to deal with.
Also, the story picks up quite a bit after the last scene of season 5, and I was pretty confused for most of the book. (The second volume goes back to just after the series end. While it wasn't any more interesting than the first volume, I think the order ought to really be switched. I spent both books confused rather than neither.)
The best part was probably the art. It really harked back to the visual feel of the series itself, it wasn't hard on the eyes like some graphic novel art is, and I just enjoyed looking at it.
SPOILER: Everyone lives. (Although some by a skewing of the definition.) So, the ending of the series didn't really matter. While some of the things they thought up for the graphic novel were pretty clever, they took away what the ending was supposed to be ABOUT. Not Fading Away. Not backing down, no matter what. Going out swinging, even though you know you're going out.
It was worth reading as an Angel fan, definitely not a waste of time, I just prefer my own mental image of the season 5 aftermath. I recommend it for Angel fans, but I suspect that anyone else might be very confused. For a really amazing Buffyverse graphic novel try Fray; I loved it even when I didn't like graphic novels and didn't know much about Buffy, and I still love it now.
Buy Angel: After The Fall