Ace of Killers is full of Aces! 4 Stars.
Forgive me if I am misremembering this, but Garth Ennis was no longer on Hellblazer and it was Paul Jenkins on the book at the time. If I remember correctly again, Preacher was still young and "The Preacher formula" was not perfected yet. If both bits of memory serve correctly, then it makes sense that this story feels like a narrative between what Ennis did in Hellblazer (making John the King of Cons and master of messing with Demons) and what Preacher would be (over the top middle finger to God and nature).
This issue is not all about John, John was only is a panel. This arc deals with a demon wanting to Kill the Hitman and the Hitman getting desperate on how to get out of the do or die situation. There are a lot of great cameos done very well in this book, such as Jason Blood (The Demon) and of course Catwoman, who you can see on the cover.
This issue had a lot of violence, but not Preacher level. The humor was pretty fun, I must admit. Overall, I enjoyed this issue and I think each character is represented nicely, even the use of Jason Blood felt right.
The Hitman stuff is slowly being collected and by the time you read this, there should be a trade that collects this arc, so feel free to read it in trade or in back issue. This is not a must for a John Constantine fan, but its a nice addition.