Particularly cool is the ability to add a second player while you're playing, just by pressing F2. The two of you can then collaborate, each controlling one monkey. Visually, Monkey Brains does about as well as can be expected, although it is unfortunate that screen resolution seems hardwired for 640x480. But while the monkeys themselves may be tiny, they're amazingly well animated. Another very nice touch is the way tree branches swish around when monkeys are climbing through them.
Sounds are good too.
The biggest failing of Monkey Brains is that it can occasionally be frustrating owing to imprecision in the design. For example, a visible cliff edge should correlate more accurately with the exact point where a monkey will fall, rather than jump successfully. Also, it's much too easy to walk through a teleporter when you're trying to pick up a chicken. If you like the game at all, you'd be more than willing to pay the small incremental fee to get more levels. Oddly, however, the game is so far available only in monolithic form.