I just last night bought Axiom Verge.
It is either a Metroid knock off, or a Metroid homage, depending on your taste. But either way, if you are a fan of retro gaming, and a fan of the original side scrolling, vertical maze, unlockable puzzling, of the original Metroid, then this thing is an absolute no brainer for 20 bucks on PSN.
From what I read, it opens up to be a little more diverse later on, but in the early going where I'm at, it's classic Metroid game play, right down to the crawling on the platform enemies, and the bad guys dive bombing from the roof. It allegedly opens up both enemy wise and in terms of weapons later on.
The reviews seem to be sitting around 4 outta 5. And the short hour I got to play last night was awesome.