Length and untrustworthy difficulty are the only minor problems here. You can finish the game in four or five hours, depending on how often you get killed and have to retreat to the start of a level with a continue. Length seems padded, too, because you only have three continues per game, and when they're gone, you have to start from scratch. This is a great game, but there are better deals out there for $15. Difficulty sometimes soars in odd places but then drops to cakewalks in boss battles.
You can run into stretches of murderous jumping puzzles, die repeatedly, then make it to the next boss and kill it in one try. This is a classic retro platformer--one of the best released in some time--even if it is a little pricey.