Well, let me put it this way: It will take me longer to write this review than it took me to play (and beat) Doeo for the first time. That should give you an idea of what kind of game Doeo is -- if you're looking for a complex, deep, or challenging game, go somewhere else! But if you want a fun, silly way to waste a few minutes (and I mean a few minutes), it'll do.
Anyway, the basic concept of Doeo is ridiculously simple, and if you enjoy Whack-a-Mole, you'll find it pretty familiar. Doeos will pop up, and you have to touch them with your mouse to -- catch them? destroy them? banish them? It's unclear. Anyway, you have 40 seconds to touch either 100 (on easy) or 200 (on hard) Doeos to proceed to the next level; after five levels, you'll battle the king. That's really all there is. Well, easy mode only features pink Doeos, while hard mode (to compensate for the higher total required) adds green Doeos, which are worth more points.
The design has a very Japanese aesthetic to it -- the art is cute and cartoony, and the background music is poppy and enjoyable, making this game a pleasure to play. Still, it's so simple that it just doesn't have much long-term value. I suppose younger players might find it more entertaining, but I can't imagine going back to play this over and over regardless of my age.
One note is that on a slower computer, I guess because there are a lot of Doeos on the screen, hard mode is embarassingly easy. However, the game is still not particularly difficult even on a fast computer; it just requires a few tries rather than one.