Why they didn't market this better at the time is probably the single most commonly asked question in gaming. It is, quite simply, the best platformer ever to hit the PC (despite not actually being a "platformer" per say) and is, without doubt one of the most original, ingenuitive, and downright fun games ever released. Some of the levels - for those who have played it the 'Lungfishopolis' and 'Napoleon' levels come to mind - are without question the most brilliantly thought out I have ever seen in a game. The gameplay is fun, quick to pick up, and hits the right balance of challenging-without-being-frustrating. While the opening section (basically a tutorial section of sorts) can seem a bit repetitive, once the game gets going in earnest it is both compelling and addictive. Buy this and I guarantee you will replay it once or twice a year!
