The graphics, music, and puzzling are all top notch. Levels 10 and 18 gave me the most head scratching, and level 19 was sort of a head-slapper when I realized a logical truth that made it much easier to approach. My only slight gripe is that on level 20 I started to notice some lag playing in Firefox on Windows on a laptop that doesn't usually lag even with the graphics-intensive games from Low Poly Day, not enough to make me not want to play it, but if it got much worse then it could be an issue.
Edit: It was lagging slightly from the very start of level 20 and throughout it. To see if it's anything else I might have running, I switched from Nymble 2 to 500 Caliber Contract without closing anything beforehand and it had a couple of brief pauses when it rendered something for the first time but not constant lag throughout (and 500 Caliber is the most graphics intensive game I could think of on NewGrounds).
And I just tested it on a chromium browser, and it doesn't have any noticeable lag, so it might be a minor firefox issue and probably not worth worrying about if that's not your ultimate target platform anyway.