I'd say Echoes was a fantastic follow up to a fantastic game.
The difficulty got to me though. I finally rage quit after getting to the
last hit on the Spider Guardian and that fucker killed me. Brutally punishing, the save points are far too few and spread out all throughout the worlds. The save points are actually good for normal progression through the game but if you back track and explore to find power ups and die you're screwed.
The multiplayer mode I think is actually really underrated, it did a great job of capturing the Prime feel but not exactly the Metroid feel.
It could have been way better if they had implanted it into Prime 3, too.
I agree it had a great sense of mystery, but not as good as Prime's. I think Prime excelled at that because it never really explained anything up front and there was just so much to scan, and thus made it feel like even more of a mystery rahter than an action.
It is insanely difficult and in many ways cheap, but a true testament of Retro's design skills is that almost all of the difficulty comes from the enemies' puzzle-like nature.
@Roph:
Super Metroid, Prime or Fusion. Don't start from Echoes or Corruption, Metroid 2 is kind of boring and Zero Mission is like a remake of Metroid 1 IIRC. I'd personally go with Super.