The basic review: the game is good, it has a lot of replayability and the potential to be responsible for hours and hours of fun. I would say that the price is a little too high (but this is debatable, the game has a ton of features, and I mean a TON), but now that it is on sale it is a very good game to have in your library.
The more advanced review: The game is a side-scroller shooter with roguelike elements (after two stages you can choose a perk that accompanies you for the rest of the run). The enemies have a health bar that you need to drain in order to defeat them (with more than one shot unless you have, like in my case, the crossbow) , and they all have their own AI and special attacks. Every two stages one encounters a boss, who represents a unique challenge, using different special attacks and patterns of engagement. The world of the game is creative and full of color, reminiscent of the one in the Stickmin series.
There are tons of weapons, some of which have special effects, like homing bullets, explosive bullets, triple bullets, all of witch are limited in amount and, despise appear to be a ton, they can run out (in other words: you need to have good aim).
They are a lot of characters to choose from (once you do some stuff to unlock them), and they all have a special ability, like a flashbang, a lasso, or a bomb (to name a few), these are things that can give you an edge or take you out of a sticky situation.
There are even tons of "mods" that can be applied to the runs, like the difficulty that goes from hardcore to EXTREME, and a ton of other stuff to play with, once unlocked.
Overall, the game is a ton of fun, and I will be playing it in my free time in order to beat it and unlock more stuff. Do I recommend you to buy it? Well, yes, it's a ton of old school arcade fun. If you like side-scrollers go for it, you will not be disappointed.