It's impossible not to compare this to Hearthstone or Magic, but I think MMDoC can hold it's own against these two other giants of the CCG scene and here is why.
Comparing it to Hearthstone:
-MMDoC is much deeper strategically.
-You are rewarded with a lot more packs by just playing the game.
-Much greater card variety.
-Three formats to play the game, open, standard and weekly (the latter is a bit gimmicky, but can be fun)
Comparing it to Magic Online (Duels of the planeswalkers doesn't really compare as it is just a dumbed down experience)
-Much better UI
-Can get new cards by playing the game
-Free to play
-Much more newbie friendly
It's a shame that Warcraft and Blizzard have a much bigger fanbase that most people looking for a Free online CCG are going to gravitate towards it without even hearing about this game. Admitedly, I've always thought the art in Might & Magic games were a bit generic, but the artwork on the cards are excelent.
It may not be as deep as Magic the Gathering, but as a computer game it provides a far better experience.
If you have an interest in Card games, do check this out!