I have played a bunch of this and have got to the point where I thought I could write some impressions.
This is one of the most polished games I have played in a very long time, and fits perfectly onto 3DS alongside Nintendo developed games. I honestly mean that. The game is an absolute joy to play, the controls are brilliant, you can move Rusty around the levels exactly as would like to; his movement is excellent, how you can easily wall jump, run and jump, change direction and so on. The animation is beautiful, and YES, it runs at 60 frames per second WHICH IS REALLY IMPORTANT! I can't stress that enough, its dreamy to play and makes the difference between an awesome platformer and an alright one.
I don't know how big this game is. I have played between two and three hours roughly so far, but I don't want it to end! I am guessing it's fairly short which fits nicely with the pricing of the game. Also, although I would love for this to be epic on the scale of Super Metroid (and I honestly think it could be, or hopefully will be one day!), I wouldn't want the game's quality to drop as a result of the scale of the game going up. The age old problem huh?
Anyway, I am very happy that these guys have found a home on a system with controls, as something like this just wouldn't work well on a smart phone (where the developer originated on, I believe), and I really hope they stick to 3DS, or any handheld/home system!
The music is great, the characters you meet are cool, I like the teleport system, saving is done well (although I wouldn't say no to a save-anywhere function as its a handheld title, but whatever), the upgrade/shop system is balanced nicely and the upgrades/new gear is great and lots of fun (risky dynamite!) Overall the whole package is very high quality. Love it.