If you have a Steam account, there is a free demo that has all of the tutorials/challenges & the first of five purely offline singleplayer campaigns. The singleplayer campaigns (IMO) improve as you go along, but the first one is representative.
With respect to replayability, if you're playing purely offline, it depends what you want. I don't think the "realms" have much replayability: once you've got "gold rank" in all five, I doubt you'd feel like replaying them. I'd strongly recommend you try the demo. << Ah, I just realised the game is on sale, UK£2.39 - Even for just the five offline realms and tutorials, that's a bargain.

If you like duels vs various arrangements of bots, you have a ton of combat replayability: the random aspect of the game make for a lot of variation from battle to battle. I've played the game *a lot* and 1v1 bots don't present a challenge, but I enjoy 2v1 and other setups.

If like the realms sufficiently to go online and connect to Snapshot's servers, there are a good number 20-40 I think (and more coming) of online, player-created "realms". These can be played single-player, co-op, or PvP (you choose).

Thanks for your reply, RafiRomero.
Yes, a gog mail popped up telling me on the game being on discount (as I have it on my wishlist) and the price seems very good.
As for the user created realms: are these downloaded and permantly added for offline play?

At the moment, you need to be online and connected to Snapshot's servers the whole time you're playing the user-created realms; this may change (you could try asking on the official forums). The game is still being actively developed, although Snapshot are currently focused on getting Phoenix Point (their new game) off the ground; focus should become more evenly divided once this happens.

I see, thank you. I'm a bit bothered since it seems that there is no x64 version available on GOG (for equality's sake), so I'll think about it.
Thanks for answering all my questions :) .

I've heard people complain about this, but I'm confused. The game is tiny. I don't think you need x64 unless you're running stuff that needs to access a large amount of memory? Or is this a Linux thing? I don't think there is anything Machiavellian going on. My guess would be that Steam require both, and GOG don't, and that another build is another thing to test, but don't quote me.
As someone on the Steam thread pointed out now, the game costs around the same as a UK sandwich now, I'm not sure how excited I can get about any of this stuff (I'm a long-time player, however).

Oh, I understand perfectly. It's not a price related thing (I mean, the price is soo low that it's an outrage by per se, for how convenient it is), the issue is that most of the time GOG versions of games are not treated on the same level as steam counterparts (missing updates, content, and so on), so I kinda hate to give money to somebody (be it a small team or not) that doesn't treat gog versions the same way as they treat the steam ones.
Steam version doesn't require at all both version, they are available from the start (and you actually use the one for your right os).

Understood.
The GoG version has never missed an update, although the GoG update does lag behind the Steam update by a day or so (I believe this is something to do with how GoG deploys updates vs Steam).

Simply put: 90% of the game is online play.
Even worse than that is the fact, that when they release an update the old version stops working, so it's Always- online -DRM.
Though I'd love the game if I could play offline, I regret buying.
Absolutely not recommended for the singleplayer content!