For US$3.99, SpellTower (now on the Mac App Store) provides an amusing diversion for anyone who likes word games. Although it's similar to other "find the word" board-clearing games, it adds in just enough bonus features to differentiate it from the pack.
The blue and black squares provide extra destruction powers and "minimum word size" indicators nicely ramp-up game play as they force you to build extra-long words. Words of at least 5 letters supply extra board-clearing power. These extras combine for a quicker game pace tempered with a few extra challenges.
Game Center integration and multiple play modes (tower mode, puzzle mode, and rush mode) mean you won't bore from gameplay any time soon. I preferred the Tower Mode (no pressure, 150 letters, max out your points) to Puzzle Mode (think "Tetris," adding a row each time you make a word). Your mileage may vary.
All in all, I enjoyed giving SpellTower a whirl. It's light, it's fun, and I don't think you'll feel cheated by your four dollar outlay.