Excellence
(note that I consider 5 an "average", and a 9 is something truly rare and exceptional)
Going into this game, I was expecting something cheap and short-lived. I didn't think I'd be playing it for any longer than five or ten minutes.
But as I got deeper and deeper into the world of Snailiad, I realized I just couldn't quit. I became almost obsessed with finding all of the power-ups and Isis Fragments (still haven't accomplished that, though I feel I've found every last power-up). Finding all the secret areas had me in a fever for a while, I think. I actually had to force myself to take breaks - few games can put me in that kind of state. :P
Took three hours to beat the game. I was tremendously frustrated with the last two bosses, spending about 45 minutes to an hour apiece on both. And here's the part that had my palm glued to my face - after finishing the game, I realized I'd been playing on Normal when I thought I was on Easy. XD The challenge was immense, I have to admit. But once I learned how to use the Snail Shield effectively against Moon Snail, it didn't take much longer for me to finally triumph (there's nothing better than beating the hardest boss in the game with just one last heart left right as one of his attacks is about to hit you). :D
I'm going to keep playing and see if I can find this Isis Room. And you should make a map showing all the secret areas with the power-ups and Isis Fragments marked on it. I really want to see all these easter-eggs, after all, but just don't have the time to look for them. Perhaps you could make the map an unlockable (or activated by the Konami Code, just for lolz). And what's this about a secret boomerang? I think I already found the boomerang (I have one, after all), but that one snail only just started talking about the thing.
Anyway, moving on to some constructive criticism before I forget. The artwork wasn't bad - far from it, it was pretty good and simple. However, the opening cutscene does need to be a bit better - some potential players who would otherwise the game might not play it just because the first cutscene doesn't grab them.
As well, the gravity-changing jump feature made things really, really annoying for most of the game. I only got the hang of it AFTER I'd beaten Gravity Box. Changing directions mid-air was really tricky - sometimes Snaily wouldn't change direction even when I was pressing both "Z" and an arrow key at the same time, and sometimes he would. Having to hold the direction you want to go BEFORE hitting "Z" was much more reliable, but incredibly hard to do when you're trying to change direction quickly. And even when I pretty much had the hang of it, trying to fight Moon Snail and Giga Snail resulted in a lot of accidental gravity-changes, which typically got me killed. When you're under stress and jumping around a lot, you'll often hit "Z" multiple times in quick succession trying to jump just once, and you'll probably be holding an arrow key in an attempt to direct your shots which results in gravity-changes galore. (though in the end, I figured out that it's best to fight Moon Snail from the floor and move as little as possible)
Oh! And I should mention that I've never played Metroid. Ever. I think this is one of only three or four adventure games I've ever played. Adventure games generally aren't for me. That said, I'd play a Snailiad sequel.
Boss Tips:
First Boss - you can stay in the bottom left most of the time, even on the ceiling right above the door.
Second - stay in the opposite corner when he releases his mini-squares. Deal with those before anything else.
Final - stay on the floor as much as possible, use Shell Shield on his cluster-attack. Keep up a constant stream of Devastator blasts and always stay facing him to stave off his own stream of attacks. Move only when you need to get under him. For Giga Snail, stay on the walls during his transitions between phases and use Shell Shield during his Red Phase (stay on the wall and don't move, just Shield quickly). When he uses his ZZZ Attack, get on a wall and use Shield again.