Ranked in these QuestionsQuestion Ranking
Pro Free to play
You have access to all levels and difficulties and the 4 starting heros. New heros can either be bought or unlocked by earning defender medals through mission completion and daily quests.
There are hero costumes which can't be earned in game but they are purely visual. The only advantage a paying player can buy is more bag space or extra hero card slots (you get +1 card slot when you unlock a hero with in game currency but can buy more if you want duplicates).
Pro Constantly being improved
There are updates nearly weekly and always new levels and characters being added. The more recent Incursions have also been more of a challenge compared to some of the original content.
Pro Visual appeal
Charming characters, vibrant colors, and an art style that is cohesive across all their content.
Con Still in development (as of 8/25/16)
Each patch often brings as many bugs or balance problems as it does bits of additional content. They are usually worked out but it might turn away players looking for a finished product.
Con No challenge until the highest difficulty
There is a campaign mode but it's far too easy even for brand new players. And after you're done with the campaign, your fastest method of leveling to cap is running the same boss repeatedly on a low difficulty. Why have 8 difficulty levels if you encourage your players to skip half of them through poor balance?