I enjoyed Keith Courage. While it is a challenging game, it is definitely beatable if you nail down the boss and enemy patterns. I thought the underworld levels were awesome. Those levels have some unique looking enemies. I especially liked the one with a huge pistol attached to its body. Controlling the mech is easy and lots of fun. The overworld levels are bland indeed, but you get over it. The only thing that really irritated me was that the weapons cost lots of money. In level 7, you literally have to spend 30-40 minutes killing enemies in order to get some cash for the Alpha Sword. All in all, Keith Courage is a solid game and doesn't deserve the bad rep that it gets.
Its strange this game did not get much respect. For a pack in game, I actually really enjoyed this one. The upper worlds, yeah were kinda bland. But.. once you enter the underworld, it really takes off. Not unusually hard, and a really enjoyable experience on the TG-16.