I've never understood the appeal of the Metroid Games - what am I missing?

You are missing a lot! IMO its the best sci-fi platformer.

I had great fun playing this series, some amazing boss battles, unlock new weapons and abilities gaining you access to new areas of the game

Rock'n'Roll

Good Game 7.7/10, Completed and Owned

I play this game with Project M 3.6 on a regular basis.

Easily the worst smash game. But still really fun to play in a casual context. Melee, Smash 4, and 64 are all way better though.