Pretty interesting.
I'll start with what I like:
-The animation is smooth, which is a big plus.
-The music is fitting and well done. Random generation is a plus as well.
-Isaac is a sympathetic character.
-I saw Isaac's doodlepenis.
And what I don't like:
-I don't like that Isaac's head faces me no matter which direction he moves, that seems quite lazy.
-The simplicity of the art style is too basic to make it unique or recognizable like South Park, Simpsons, or even Dan Paladin's basic-but-identifiable style. Instead this seems to be common stick 2.0.
-"3+ hidden characters"? 10+, 100+, even 5+ I suppose makes sense but when you use a weird number like 3 and slap a + on it, it usually means "that number + 1 or 2, but we want you to be way more impressed than you should be." I'm not a fan of cheap marketing attempts.
-Horribly disfiguring Isaac with cybernetic implants is fun from a gameplay perspective, but after establishing him as a character to feel sorry for this just feels horrifically wrong.
-I saw Isaac's doodlepenis.
As a final note, I realize we all have to get by somehow and I certainly wish you the best of luck with selling your game. I hope one day to market stuff on the internet for a little extra pocket change, so sincerely I can relate. However, I think sites like Newgrounds may make this difficult since many games of roughly the same quality level as this one appear here for free. Overall, I find this game to be intriguing, and though I laughed at the thought before, I may end up purchasing this game at some point. You should probably market it as only costing $3+ though...