I have downloaded and been playing Wonder Boy in Monster World for awhile.
The game is fun, but has its flaws.
It feels most like Zelda 2's Side Scrolling levels. In some ways it is more polished, and others less polished. The game isn't overly hard, but it does have a slightly unforgiving collision detection, and its shield isn't as easy to use as you would hope.
Overall though, the spirit of the game is great, and the game is fun.