I've been playing Hex for a long time; the community is pretty active and stable since the Kickstarter did such crazy business.
I'd say it's not really viable as a F2P game even though you get a few hundred cards free when you start out: think of it more like "what you wished Magic The Gathering online would be, if you wanted something like that" which also means the payment stream is fundamentally CCG. If you want to play, you're going to need to sink a bit of money into it. Getting mess-around decks or decks that can handle the PVE / Single Player content will be pretty cheap (and you *might* be able to F2P that even), but if you want to be viable in P2P you're going to need to pay for card packs. I'm never a fan of "constructed" PVP in CCGs anyway, I drastically prefer "Sealed" or "Booster Draft", but doing that consistently free is going to be damn near impossible, and that's where you'll buy a lot of your cards (3 packs for booster drafts, 6 for sealed). There is an auction house and 3rd party sites for the buying and selling of individual cards, but I haven't used either yet.
Basically, think of the "free" part of the game as a demo that lets you get familiar with the game, the client, and all that stuff to decide whether it's something you want to put some money into or not.
With that said, If you want an online CCG, Hex is *great.* It's very different from Hearthstone; much more like MtG in terms of complexity and interaction. The PVE stuff has been significantly slower in rolling out than anticipated, but overall the game is fantastic with tons of strategic variety. It definitely apes MtG pretty closely, and you can even use similar deck archetypes if you're familiar with them (red burn, green ramp, blue control, etc), but they also do a good job of easing you into things if you haven't played many CCGs before.