It's hard to imagine a simpler casual game than Fruit Ninja. It's the perfect toy for when you have just a few minutes to kill, and you don't feel like thinking. I don't mean to denigrate the game--frankly, sometimes a little mindless distraction is exactly what I crave. And this one is pretty fun.
In Fruit Ninja, various whole fruits are lobbed up from the bottom of the screen. You must slice them in half (by swiping your finger across them) before they fall off the screen again. Really, that's pretty much it. For each piece of fruit you miss, you lose a life. If you lose three lives, it's game over (though you can get a life back through bonuses). Adding to the challenge are bombs tossed in among the fruit, and if you accidentally slice one it blows up and you get an immediate game over. You earn bonuses for cutting three or more pieces of fruit in a single slice.
The game offers incentives to keep playing: As you go, you can unlock different blades (some sparkle, some flame) and different backgrounds, just to spice things up. You'll also find a Zen Mode, where there are no bombs and you simply have 90 seconds to slice as much fruit as you can.
The animation and sound effects (should you turn them on) are all very good. The graphics are smooth in a cartoony way. The catch is that the game's responsiveness is dependent on your phone. If your phone has a sensitive touchscreen, you're golden, and your slices will be very accurate. If your touchscreen isn't so great, or if your phone has been getting a bit sluggish, the timing is liable to be a little off, or slices won't be as long as they're supposed to be, and you'll miss fruit that you should have hit. That can be very frustrating, but it appears to be a problem on the phone's end, not the game's. It will certainly work better on faster phones (it's gorgeous on the Droid X).
This game really doesn't have a ton of variety, but if you want a quick, casual distraction, Fruit Ninja fits the bill nicely.