Kirby 64 is one of my favorite games. It's a simple game, but the level designs are so creative. If you don't collect all the shards, it's pretty simple. However there is so much re playability, because you can go through each level with a different combination of power and some of the shards are no easy task.
If you played it how I played it as a child, I say there's is 20 hours at the least of gameplay.
I really recommend this title.