Downloaded the game this afternoon and played up to the third world. Really enjoying it so far. Game controls well, sprites are great, music is cool, all that good stuff, etc. Collecting all 100 Diamonds per stage is definitely addicting. I also like how there is some degree of backtracking by exchanging Diamonds for upgrades to access secret stages.
Just a question about the one upgrade I got so far. I got the gun upgrade and it allows you to access those hidden walls, but does it do extra damage to regular enemies? it doesn't seem like it's doing anything. Is it firing faster than normal and I'm just not seeing it? Kind of weird to have an upgrade that ONLY allows access to secret stages. Would have been cool if it did more damage. Could have given it a cool down rate or recharge time if the devs were worried about it being overpowered. Whatevs, though, not a big deal.
Only other gripe I have is that there are no bosses. Platform/Action Adventure games, fuck it, every game NEEDS bosses. There really should have been boss doors on the hub map. Other than that the game is definitely worth picking up if you like 2D platformers.
Oh, one thing I also wanted to mention is how short the stages are. I always worry when I play these non-Japanese/Indy-like platformer games that the stages are going to go on forever. A bit unrelated, but Henry Hatsworth was a platformer that had a lot of the right ideas (just like this game), but the stages never ended and there was just no sense of design. Thankfully, so far the stages are compact and concise in Mutant Mudds.