If you want the short version, skip to the bottom.
The game is relatively fast-paced due to utilizing a simultaneous turns mechanic, with both players completing their actions at once. This establishes a very engaging experience in which both players have to try and guess at what the opponent will do, and how to respond. The game also allows for a great deal of strategy, utilizing both a "health" system akin to Magic or other TCGs, as well as a morale system (which effectively functions as a second health meter, except it drops from losing troops in battle), multiple combat zones, and an initiative system. There is definitely plenty of depth to be had in this game.
The game is also very good about rewarding every portion of your progress. There is a leveling system which gives you free cards when you level up, and you also get free cards from completing various achievements (termed "quests" in your profile window) such as beating the AI in training matches multiple times, completing the (very short) campaigns for each faction, and beating Academy challenges. And the game even allows you to construct your own decks for facing off against players or AIs.
So why am I not recommending this game? Well, I should be honest. This is perhaps one of the first times I'm actually NOT recommending a game, it's simply not something I do often. So I want to be perfectly clear: this game is fun, but there ultimately is one MAJOR flaw with its design structure.
No, before you say it, it's not that the game is Pay To Win. On many occasions I actually beat players using mostly starter-level cards. And some players have done quite a lot with their starter cards and worked hard to become top players. But with that said, the community needs to be more honest with itself: newer, free players are definitely at an inherent disadvantage compared to players who have spent lots of time and/or money on the game.
I'll try to explain, since that sounds like it may contradict what I said before. Again, I'm not bashing the game, and it does make some effort to give you lots of cards to play around with. But it's not nearly enough.
In my experience, completing every single campaign mission and PvE fight possible only leveled me to just below Lvl 9 (there are 50 levels). So it's relatively clear the game is trying to encourage you to face other players. I had no problem with this, so I started playing some matches against other people. But I quickly began to realize there was a problem: they had WAY better decks than I did, filled with cards that were in many ways significantly better than mine....and I had no way to prevent them from beating me into the dirt, no matter how well I understood the game or how well I played.
I can recall several matches in which I played my deck's strategy out perfectly, and even outplayed my opponent for the majority of the game, only to fall behind and eventually lose because they kept pulling out cards that I'd never seen before, cards I had no access to, and cards that I had no defense against. And yes, I can tell the difference. I played several folks who were just a lot better than me....but also a number of people who clearly were not, who ended up beating me anyways because their deck was full of cards I couldn't combat.
I recall one particularly egregious match where my opponent was being repeatedly outplayed card for card. Every time he made a move, I was predicting and countering it. I got him down to a mere 5 HP, while I still had 86, and I had multiple creatures in play with Flying (he had nothing that could block Flying). So imagine my surprise when he started employing cards that I'd never seen before, turned the match around within a few turns, and ultimately won. Not because he was "better", mind you, but because he was using cards that I had never seen and had absolutely no hard counter for amongst all of the starter and booster cards I'd received from PvE content to that point.
I made an honest effort at the Rift system, too, but ultimately it's got that same problem, but worse, because the players in that mode are clearly superior to the ones in normal queue matches. The gap between a newbie and a veteran is simply too great to overcome, even if you get pretty good at the game relatively quickly as I did.
Now, let me return to a previous point: the PvE content only gets you to around Lvl 8 or 9, which doesn't yield you nearly as large of a selection as you'd think in terms of cards to build your decks with. And to make matters worse, while the game does allow you to buy booster packs for free (using an in-game currency obtained during matches), the booster packs are prohibitively expensive. 15 cards costs over 5k points....and running all of the PvE content in the entire game only netted me about 5.5k or so, and losing a PvP match nets you less than 100 points per match. So in order to gain enough points to buy a 15-card booster pack, you have to effectively run over 50 regular PvP matches. Which is by no means reasonable.
I've seen former and current players who even admit to this being a major issue with the game, so I know I'm not imagining it. One particular testimony came from someone who (at the time) was a Top 10 player. He admitted that he got there by spending lots of real money to build a good deck, and after that, he was beating people right and left.
SUMMARY: No, the game isn't technically P2W. You can earn cards for free if you have the patience. But if you purchase cards with real money, you are saving yourself countless hours of grinding, and you ultimately WILL perform much better than you would have if you'd tried to play with just the freebie cards that the game gives you. No amount of personal skill or TCG experience can overcome the significant gap in deck strength between a veteran and a new player.
You will either pay for better cards, or you will have to grit your teeth and lose a *lot* of matches in order to earn them for free. I chose the third road: After several matches against opponents that I should have beaten, but instead lost to, I finally gave in to my frustration, closed the game, and uninstalled it. The core game is a lot of fun, but it's mired in the "grind" of obtaining new cards and the game's significant balancing issues. If the above stated issues are resolved at some future point, I might be persuaded to return at that time. For now, it's a no-go for me.
EDIT: Be warned! A quick browse of the Steam forums shows that nothing has changed in the month and a half since I posted this review, back when the devs promised that "internal discussion" was ongoing about how to fix the issues I've described.http://steamcommunity.com/app/257730/discussions/0/610573751153594480/http://steamcommunity.com/app/257730/discussions/0/610573751148628044/http://steamcommunity.com/app/257730/discussions/0/610573009256995538/
The game is still excessively grindy, the matchmaking is still poorly executed, and the devs are continuing to promise future changes with no ETA on the horizon. The devs have also made statements in each of these threads which have me believing that nothing will change in the long run, as they seem perfectly okay with trained veterans beating up on tenderfoot newbie players and are resistant to the idea of raising XP/IP gains to reduce the grinding. They've also admitted that the game has far more veterans than new players, which means that they're having retention problems....probably due to the exact same issues I've expressed, the issues that drove me away from it.
Until the devs demonstrate a willingness to fix these issues, I will continue to recommend that players stay far away.