"Not again, another pay-to-win!" is what I thought at first as well, but I've managed to appreciate how strong are the custom (and common) cards. I've just beat a guy with like 700 rating points (since I just started out, I was ranked no more than 100) with ONLY the starter cards and some quest awards. I've used NO extra units, while my opponent obviously did - some of his cards (and bonuses) were completely new to me. So yeah.
And though I finally decided to throw a few bucks at the creators (being a [beginner] game developer and understanding the pain), I don't think I've gained any "unfair advantages" over the non-paying players. Well, I've managed to create a deck NOT based on gold, but that's about it. What's funny and sad at the same time, I think it's way worse than the starter deck. Really, I kind of wish I've never bought the boosters or - at least - not of this type. Either I'm unlucky with the cards or inexperienced, but I think gold-based decks are stronger and easier to play/set up by FAR. And yeah, hybrid decks are way beyond me (maybe after a few more boosters? Not sure if they could actually work out - maybe I'm just missing some crucial cards).
Don't get me wrong, the game has its flaws. Gems are a HUGE pain to get (at least right after the campaigns, not sure about the late game). Probably that's why many people might think it's a pay-to-win kind of game. It's true in some way, I guess: if you want diversity and rare cards, you either grind the hell out of the multiplayer mode or.... pay. But, as I've said, the starter cards are pretty powerful and if you manage to spend your commander's points right, you might just stick with the custom deck.
Solution to the pay-to-win impression? More accessible gems, more starter decks (one for each resource?). Also, there could be a few cheaper boosters with more common cards, 200 is kind of tough to get. Even better, player could choose - let's say - 10 cards of different rarities after the last campaign to complete his deck and/or have the possibility to try out different resource army.
I just don't think that I'd like to spend a lot of time grinding in this kind of game - and I would REALLY love to create more interesting decks.
The other problem is... lagging. Well, it MIGHT be just me and my not-so-new laptop, but the screen transitions are terribly SLOOOW. Given that there are many unnecessary transitions (like exiting campaign to edit deck and going back to the town - instead of story mode - for no apparent reason), you waste a lot of players' time. I know that it's just Flash, but I've seen much more complex multiuser games running much faster. Optimise some stuff, please.
Overall, it's a decent game with interesting mechanics. While it requires some luck (as most CCG-s do), careful planning and anticipation are crucial to win. I would give it 5/5, if it weren't for the damn slow transitions and poor customisation at the beginning.
Do I recommend it? If you're patient, why not. It's a solid CCG and probably one of the few that rely mostly on skill. Trust me, to gain unfair advantage, one would have to spend FORTUNE on boosters. The game is well thought out, I have to admit.
Seriously, try it out.