I mean, it's definitely good, and a lot of fun. Sadly, it has a some issues?
Most of them are about controls.
For some reason, game seems to suffer slight slowdown at relatively random points. In another game, this would be barely noticable, but because it's a precision platformer, here it's a case of life and death. It doesn't happen often, but when it does, it's usually lethal. I can't say for sure if it's something to do with my computer or browser, though.
Second, hitboxes in the game seem to be semi-random - sometimes I touch an enemy even though i'm a pixel or two away, and other times a fireball flies right through my face without connecting. For example, in stage 17 I was able to find an inconsistent safespot on the long platform before the second spiked ball (if standing right at the very edge), which allowed me to pass it without dodging through the chain, which seemed to be original intent, and made the stage a joke.
Also, for some reason, on my computer, using WSAD seemed to have a split-second input lag, when compared to arrow keys which were near instantanous - not a big deal, since, well, solution was to simply use arrow keys and game worked fine. And really, none of the above problems are particularly game-breaking or anything.
Only real issue I'd have is that sometimes level design is a bit off - several levels in the Water Dungeon consist of simply doing exact same thing, twice - and at other points, trial and error kind of becomes a thing - but this is kind of due to the game's design.
See, screen is really small vertically, but levels are designed so that most of the time you move vertically - which means that most of the time, you can't see where you're going. Your character takes up like, only 1/14th of the screen horizontally - but 1/5th of the screen vertically, which means that you can't see any further than two lengths of your character up, or down. Which means a lot of levels involve rushed guesswork. And for the record, when it works, it works phenomenally - trying to think on the fly where to go next trying to escape rushing water or lava is a really great feeling. However, sometimes this simply results in blind jumps and cheap deaths.
Still, overall, despite some flaws, it's a really great game - it nicely recreates "bite-sized" feeling of old Game Boy platormers, which works really well for a browser game, and provides you with a good challenge that provides a feeling of real satisfaction when completed - and while it's difficult, short levels ensure that frustration never sets in and let you take game at your own pace, and you're rewarded for every difficult bit with progress.
All in all, while maybe not a masterpiece, it's definitely a really good, fun game that I came back to several times now, and I will probably keep coming back to - and I'd recommend to everyone looking for a good platformer that doesn't rely on cheap traps.