Yah, Blood and Ashes was the last set. I've only read up to part four or five, but what I read was shite. Everything Azzarello's done, Ennis has already done with Constantine, and done it better.
As to what I'd do...I'm not sure. To tell the truth, I'd want to be able to read ALL of the series first, since I've been limited to the trades so far.
One more point: I like what Azzarello's done with 100 Bullets. He should just leave John Constantine alone.