So far it is getting better with each new episode.
Episode two was also quite good, but I believe I'd find it better if I haven't watched first eight seasons of Supernatural (should have stopped after sixth though...), which brings me to my point - they should avoid setting episodes in rural areas and small towns as it seems too similar to Supernatural that way. Using Chicago and other cities would easily distinguish visual and atmospheric aspect of the series.
I still have few quarrels with the show though:
I find it to be too light, it needs to feel grittier and be more dark. There should be a feeling that beyond the thin curtain of everyday life, world is much more ugly, desperate and threatening place that Hellblazer comic so often evoked.
I don't really like portrayal of magic so far. Or to be more precise I don't like the presence of so many new magical artifacts in every episode, like those St. Padua nails and magical card. If Constantine can make things easier by pulling a magical item out of his pocket anytime, it may become a Deus Ex Machina and kill the suspense.
I'm also not really fond of Constantines narration at the end of every episode, as I don't like that style at all, but that's just my stance on it.
Oh and, lastly, though it's been said thousands of times by everyone, I must add I am disappointed that he's not smoking. Well he is, but barely and the way he never inhales the smoke or light the cigarette in the shot will sooner or later become a bit odd and distracting. It might be even better to have him drop the cigarettes entirely than cheat it this way, as it is very noticeable.
I do think that show has a big potential to become really good though. Matt Ryan is perfect for Constantine, though they have made his character a bit too likeable and hero-like. A bit more cynicism and arrogance would do a world of good to the character. He's supposed to become likeable despite being bastard or to say it differently he's supposed to become likeable exactly by being selfish and unlikeable and not by being easy to sympathize with.
When I first picked up the Hellblazer comics Constantine was such a fresh character to me, and I find this show to go a bit too much "by the book" right now, with both characterizations and overall storytelling.
Again, Matt Ryan is so much fun to watch in this role and the potential for it is definitely there, that I really hope it'll last to become better. Hopefully they won't cancel it so soon and will give it a chance to flourish.