Shifters is a fun little match-3 game with a "twist" -- instead of switching colored blocks to match three of a kind, you rotate them in sets of four. That changes up the strategy a bit, as the patterns look a little different from the standard Bejeweled-type game (the shifting mechanic is obviously more similar to Bejeweled Twist) but the gameplay is still simple and fun.
Shifters is presented in levels, too, with each level providing a separate goal to meet, from matching a certain number of shifters to meeting a score within a certain time.
Is it simple? Yes -- if you want anything more than some empty tapping and silly faces, you won't find it in Shifters. But the mechanic is entertaining enough to while away a few minutes of waiting in line or for the bus, and the 99 cent price is worth it as long as your expectations are low.