In general, $0.99 apps can be hit or miss. Some apps just seem to "get by" with the barest essentials. Plasma, however, comes with good graphics, good sound effects and a nice soundtrack.
The gameplay is simple. Plasma balls fly across the screen and your job is to destroy them with your finger before they get to the other side. If the ball crosses over to the other side, you lose a bit of your "Life". If your Life meter goes to nothing, the game is over. The game is also enhanced by red Plasma balls you must avoid. Tapping on a red ball also reduces your life meter.
Reading the game description doesn't quite do it justice and the game quickly ramps up to a frantic pace. Watch this video provided by the developer which captures the game well (and also demos the mega plasma ball that eventually appears)
While Plasma keeps track of your high scores locally, the key feature that adds replay value is the inclusion of net rankings so you know how you compare to the rest of the Plasma-playing world.
App Store Link: Plasma