Depends.
The original ongoing? Yes it's great; most writers' runs are at least good. Standouts are Delano, Ellis, Ennis, Jenkins, and Carey. Azzarello has some good issues but his run overall is bad. Also, from the moment Denise Mina takes over from Carey, quality falls off and never gets as good again. Diggle had its moments, but still not good enough. We had high hopes for Milligan, but it sucked, and the ending (to the whole series!) was bad and confusing.
So you can read from the start (Delano) to 215 (Carey) and it'll be an overall great experience. But I recommend stopping there.
The many relaunches, reintegrations into DC, and reboots of John books that came after? No, it's mostly shit.
The current iteration (John Constantine: Hellblazer /JC:H - Dead in America, by Si Spurrier) is
decent, a
wee bit of a return to form, but still doesn't rank -for me- among the
good Hellblazer runs mentioned before.
Ironically, Spurrier put out an original book called
Damn Them All! which was a better Hellblazer than his Hellblazer, and if you do some
search and replace of a couple characters' names in your mind as you read it, you can pretend it's the new ending to the original series.