(Ohhh, my god, this is became an essay.)
Worth watching/ should I watch it? * Yes. I was similarly disappointed with Arrow Seasons 2-3. I enjoyed this show as much (or more) than my favorite parts from Arrow and the Flash.
What's it like? * It's a character-driven drama. The protagonist's motivations are extremely sympathetic. Like Oliver when he started his crusade in Arrow, John is very much an Anti-Hero, though of a different kind. The supporting cast (not counting Liv) are more than they seem at first impression, and also badass. There's funny shenanigans and banter between the characters, as expected. Like Oliver and Barry, John Constantine has a sort of team, and there are great relationship-building moments. There is also some romance (or the beginnings, at least). Plus, John is occasionally really weird in a highly entertaining way (Your mileage may vary, but I laugh just thinking about it). * It does a bit of world-building, establishing some rules/existence of: witches/warlocks, angels, demons, alternate dimensions, soul trade, lost souls, Hell, Heaven, resurrection, astral projection, and the actual Devil (not Lucifer, the other one). Since most these subjects haven't been explored on Arrow or Flash, this puts certain Flarrowverse events in a different context. The parts that are pure horror are truly horrifying (episodes 04, 08, 09, & 13). There's also some fun use of seemingly ordinary items with magical properties (the prop and set design are fantastic), and quite a few magic spells complete with legit-sounding incantations (from what I've read, these people did do the research). Characters and items from the comics are established as well. * There is a mysterious, actual world-threatening Big Bad that connects all of the little battles, albeit indirectly. It follows a conflict-of-the-week plot that falls under the over-arching main conflict. Unfortunately, the main plot and certain sub-plots are ultimately disappointing, because the show was cancelled before they're resolved.
Comparing to The Flash and Arrow: * It may be more... depressing, I guess, than the CW shows. You know, going in, that the Flash and the Green Arrow are "SuperHEROES." Even if they do some Anti-Hero stuff, they're going be heroes, and, somehow, they're going to come out on top. There is a really strong sense of hope in both CW shows. With Constantine, you don't really know where the story, or the main character is headed, how much of a hero he really is, or if he's going to win. There is a sense of hope, but it's a bit muted by horror and foreboding. (In my opinion, this made the show better than the other two by far: the stakes are higher, so the victories are sweeter. But I read a comment somewhere, where a viewer complained that John was too downtrodden by the plot. To each their own.) * On a lighter note: where Flash explores crazy, out-there "science," this series explores magic. If i remember some of the earliest Arrow episodes correctly, the team dynamic is a bit like Team Arrow's when they started out (similar problems).
How many filler episodes? * Only one, if you ask me. Filler being defined as "unrelated to the main plot, don't significantly alter the relations between the characters, and generally serve only to take up space" (TVTropes). From what I can tell, people see this as subjective--based on what they personally expected from the story. Judging by this, I think the only filler episode is the sixth, "Rage of Caliban." (I still recommend watching it.) * I've read things from viewers calling more than one of the episodes "filler," which is to be expected. Most of the episodes significantly alter relationships, but are only related to the main plot indirectly. They hang on the idea presented in the pilot, that all of the conflicts share one source--one Big Bad--whose intentions are left very ambiguous for most of the story. How the resolutions of these conflicts actually advances the main plot is unclear. * I'd say at least 6 out of 13 episodes are definitely exempt from "filler" status. The first, fifth, eighth, ninth, and thirteenth episodes present more direct connections to the main plot. 08 and 09 in particular are a really good two-parter. And the fourth episode may not do much to advance the main plot, strictly speaking, but seems to be regarded by most as one of the best, if not the best episode. It is essential for establishing the protagonists' characterization.
Miscellaneous: * I should note that I really fell in love with this series reading earlier scripts for the first five episodes; I am disappointed with a couple of things they cut out, one in particular, that I thought were sort of key to the story. SPOILER * Also, a large part of the reason I watched this was to see what the hype about the Mill House set was all about. If nothing else, you might look up some stuff about that, and the props, 'cause there was a lot of work put into it and it's really cool.