I played a demo of this when I was ten years old, and I remember thinking it was great. When I saw it on GoG, I bought it right away, thinking five dollars would at least be worth the nostalgia it would provide. Turns out the game doesn't really change after the first half-hour on display in the demo.
I probably could've lived with the dated control scheme. Even the rather mindless combat (hold down shoot and zigzag) and repetitive gameplay would've been tolerable. But in a game with levels that last from half an hour to an hour, a complete lack of checkpoints or any kind of intra-level save system is tantamount to a mortal sin. All these factors combine to make MDK an exercise in frustration.
The game's aesthetics are certainly top-notch, and it's sense of humour is a nice addition, though it's never laugh-out-loud funny. But all that can be gleaned from watching someone else suffer through the game on Youtube. I'm told it's short, only six levels long. I only made it through four before kicking it off my hard drive.