More Bloons is a perfect example of a very simple concept which could very easily be a terrible game, but because of the obvious care and attention put into crafting the levels, it is instead an entertaining challenge. Probably one that will have you tearing your hair out at some points (at least, if you play without unlimited darts), but one that will give you quite a feeling of accomplishment for finishing it.
In More Bloons, you control a dart-throwing monkey. You move the mouse to aim and hold down the mouse button to set your throwing power. There's a number of balloons scattered around the level, and your goal is to pop a certain percentage of them. At the beginning, most of the balloons are your garden-variety ordinary balloons, but as you proceed through the game, you'll see a dazzling array of balloons which do various special things, some helpful (for instance, the dart balloon which sends off eight thumbtacks in all directions), some not-so-helpful (for instance, the ice balloon which freezes nearby balloons, making them unpoppable), and some which are a mixed bag (for instance, the bomb balloon, which blows up everything nearby, including your dart). There's also blocks, including metal blocks, which stop your dart, rubber blocks, which your dart can bounce off of, and wooden blocks, which can be broken by your dart (or other objects) hitting them.
Anyway, as I said, what really makes this game solid is the level design. In each level, you have a very limited number of darts, and the level is constructed so that reaching the necessary percentage to beat the level in the number of darts you have is not an easy task, and often requires a lot of careful thinking and precision aiming. (Actually, this brings up one of the weaknesses of the game. You can also enable unlimited darts mode, which does what the name says, but the game doesn't track whether you've beaten a level with unlimited darts or not. As a result, the final badge for beating all the levels is only a medium, since it is pretty trivial with unlimited darts, although without unlimited darts it is quite a challenge.)
The later levels require quite a few attempts to beat (and I admit that I had to look up strategies for a couple, since I couldn't quite figure them out). You'll often find yourself resetting a level after a botched first shot; it's kind of frustrating that there's no way to do this using the keyboard, since if you're just a little bit off you might not want to have to move your mouse. There are 50 levels in total; each level shouldn't take you too long, but the total time spent does add up if you're trying to beat them without using unlimited darts.
The graphics are pretty simple; the monkey is cute, but the balloons are, well, pretty much just balloons. There isn't any background music, and the sounds are also pretty basic, though the sound of a whole bunch of balloons popping in rapid succession is pretty satisfying.
Overall, there's perhaps a little too much frustration in getting a level right in More Bloons to make it a truly great game. Still, the very fact that the levels are that well-designed makes this game a solid challenge and a game worth playing.