If you have any interest at all in the Metroidvania genre, this is the game for you.
Since the most recent update, there are now 4 playable characters, each of whom has distinct play styles - not only from each other, but even with different builds of the same character, via the various weapons which you can use.
This game rewards perfection - after you kill a boss, you'll be judged on how well you did, with combat (amount of damage taken), time, and potions (did you run out of health/mana) all tallied up. If you're good enough to get an S or A rank, bosses will award extra skillpoints or stats, which will make your character that much more deadly. In contrast, if you barely squeak by a fight with a C, D, or F rank, you'll get a handful of XP, barely more then you would killing some no-name minion.
Each character has 4 stats (strength, agility, intelligence, and luck) and 3 separate skill trees which you can invest in, all of which can give bonuses and unlock new abilities.
In addition to the different weapons which I've already mentioned, you can also get about a dozen different kinds of armor, all of which have different bonuses; accessories, which tend to be very powerful, but all have some kind of tradeoff; and assistants, other characters who you team up with during the game who can be called into battle to fire off an attack and often give bonuses as well.
Oh, and your weapons, armor, and assistants can all be leveled up, improving their stats and often giving better/bigger/more attacks.
If I had to name some negatives, one would be that trying to max out everything (which isn't needed to complete the game) takes a lot of grinding, and that some bosses (especially some of the optional ones) are exceptionally difficult (to beat - let alone get a good rank on). Minion respawning (which takes place whenever you leave a screen) can also be a pain, although it can be used to help collect more materials for upgrading and/or gain levels.
In conclusion, an extremely strong game which has seen amazing support from the developer.