I know it's a cliche to say something is "easy to learn, difficult to master" but that applies perfectly to this game. The basic rules are very straightforward, and the starter factions are very effective introduction to them, but the sheer number of card interactions is staggering.
The goal of the game is to kill the enemy summoner, but there's a whole board and several enemy units between you and that objective. You sacrifice cards from your hand to create magic used to summon units, so position and card management are very important aspects of the game no matter who you play as.
There's many factions, and everyone will be able to find one that suits their playstyle and their preferred level of complexity. There's the straightforward attack factions, factions that require finesse and decision making, and others everywhere in between. The "Reinforcement Packs" (expansion sets) are nice additions, but every faction has a very capable standard deck and a lot of personality.
Some games can take a while (especially the 3-4 player variants), but overall this is a very well-designed game. It's easy to pick up, but for players who want it there's a massive metagame available. The customization options and the uniqueness of each faction is great even if you don't.