Not as bad as some reviewers make it.
I found the graphics the best part of this game. Those were really nice.
The gameplay started to be better towards the end.
The jumping was indeed a bit annoying sometimes, but to me what was the most annoying is that sometimes double jump didn't work if you pressed the up arrow twice (maybe because double jump only works when going downwards and I pressed it too early? Still annoying).
There was a glitch in the last level: After I finidhed the boss, I got the reward money (and the story about what the end boss said about the scroll), but the game then stayed in that level. I killed the other guys there, took everything, but couldn't do anything else but quit using the "m" button. When I did that, the game hanged, still in that level... After refreshing the browser window I was in the menu again, and WITH the reward money from the last level! I would have preferred to see an ending of the story rather than being stuck in a level with a glitch...
The secrets are nice. The stealth started to get nice once you had enough upgrades. In the last levels, I started to feel really ninja-like when I could throw shurikens at enemies while they didn't even detect me, and when using "d" walking past everyone undetected. In the beginning the stealth skills were really useless though.
I didn't understand the point of the throwing spears, maybe I missed something (low range and 1 damage even after upgrades?)
Sometimes it was frustrating when accidently falling down at the bottom of the level due to a jump annoyance and having to start over.