Cave Story was a freeware game released in 2004 that quickly gained a large fan base. We eventually saw several commercial releases in 2010 with a PC release called Cave Story Plus released 2011. It is a very fun Metroidvania game that's definitely worth playing. That being said there are a few things I didn't like about the game.
What I didn't like:
Jumping physics- Uses the Mario style of jumping where you never quite stop but slow down to a stop. I never liked this esepcially when I keep slipping off ledges.
Secret choices and closed off areas- Just like in old video games, there are secret choices and decisions that affect the ending and the weapons. The game is semi-linear like Metroid so you can revisit areas using the teleport system or travelling there. However there are some areas where you can't revisit or quests you can't complete if you make a decision so if you miss it, that's it! Just something to be aware of it. I personally don't like it and I think developers should stop doing this.
Final level, ending zone is sealed off - Once you enter the final few areas, that's it, you can't turn back after that! I wished the game would have warned me because I would have liked to have gone back and finished up some subquests.
Short- Playing on easy, I finished the game in less than 6 hours even including all the deaths. It's a good time considering this game was originally a freeware game. I wished they could have included a couple of more extra levels or areas.
Overall, Cave Story is definitely a solid game experience. I didn't find it particularly revolutionary or as polished or large as Metroid or Castlevania games. That being said, its about 75% as good as these professionally created games from professional studios. In the end, I think that's part of its success, it's inspiring to think that a freeware by a guy and some help from his friends can be almost as polished as game put out by companies like Nintendo or Konami.