Simplicity. Often the simple games can be the best. No complex rules... no complex controls... just challenging game play. A game in which you know you're improving even when your performance in each game isn't necessarily superior to the last is a key ingredient to addiction. I think City Jump hits much of this formula and that leads to a very satisfying gaming experience. And it's free.
City Jump's premise is simple. You're a super hero (with the original intent of being Superman based on various clues in the game) scaling the walls of buildings on your way to ward off danger. Several obstacles stand in your way. This is best illustrated in the screen shot. In essence there's a building on the left and on the right of the screen. You're on one of the two sides (on the left in the screen shot) and can jump back and forth between them. As you scale the walls you are vulnerable to enemies. When jumping you are also attacking and can smash through and eliminate enemies for points. Should you eliminate three identical enemies in a row you power up and become invulnerable for a short period of time racking up even more points. Sounds easy, but it's hard. This actually reminds me of a simplified Doodle Jump that doesn't lose any of the fun in the conversion.
The graphics are pretty good and the frantic music that plays along with the drop off upon your demise are excellent and really add to the mood of the game. Even better... controls consist of simply tapping the screen when you want to jump. This is a game you can play in one hand with your thumb. As stated the game is free as in 'ad supported'. There aren't a lot any options for customizing your experience in terms of difficulty or anything and no, this isn't a game I play for hours on end. But this is a game I've been coming back to several times a day for more abuse.
Conclusion? I love it. This would be well worth $.99, yet it doesn't cost a thing. The advertising that pays for it is unobtrusive, and it's an ideal mobile "pick up anytime you've got a minute or two to play" game. Go get it!