Fun and Interesting
Very good game in my opinion, but it's lacking a bit of 'Shazam!'
Here's what I would add:
An interactive menu with a Play Game, Instructions, and Credits buttons.
A looping mp3 for the menu.
A small sound clip for a completed/failed level. (The game gets a bit monotonous after a while of no sounds except for the occasional 'Zap!'
Make the bonus level check and see if you've typed the statement in correctly so that the player doesn't have to sit and wait 10 seconds before starting the next level. Also, duting the bonus levels, don't allow the player to copy the phrase (Disable right click, or don't use a textbox). Also, make it so that more points are awarded the faster that you complete the bonus level.
The only mentionable error I found was that 'Letters Remaining:' changed when the oncoming letter turned green instead of when it disappeared. I don't know if that was intentional or not.