Simple, but kind of fun.
I was skeptical about this, because when I saw the title "Penguins Can Fly", I figured it would be a clone of Learn to Fly (or L2F). But it's definitely not that, and it's actually a pleasant little game. It does have a similar "upgrade store between levels" mechanic, but that's common to a lot of games. Also, I can't complain about having more penguin games, because penguins are just adorable.
I do have a little constructive criticism, though. I don't want to have to go through the level select after every stage--I'd rather have a "next level" button. Also, you could have gotten the user name from the NG API and used it as a default, instead of making the player type it in. Level design was good overall, but this game is a bit too easy for me. (Perhaps it'd be perfect for the target audience--I have no way of knowing.) Maybe winning the game could unlock harder levels or something?
Anyway, cute music and graphics. 8/10