Okay, this is what I picture (with the names changed to protect the distracted):
Mom: Hey, Johnny, here's my phone to time you doing your homework! You can try to get your homework done in 30 minutes and beat the clock!
Johnny: Cool! Thanks, Mom!
(20 minutes later)
Mom: Johnny, how's that homework coming? Did the timer help you manage your time?
Johnny: Uh, well, I just thought I'd take a little break from my homework and play Angry Birds on your phone. I guess I got a little distracted.
Mom: Darn that colorful Time Timer!
I'm thinking that if your child is easily distracted and tends to dawdle, the Time Timer would be good in theory but not in practice. If your child is very self-disciplined, they might be able to resist the allure of the shiny phone with the timer, but then they wouldn't really need it. (If you can lock everything BUT the Time Timer, it might work. But then again, my kids would probably spend the 30 minutes playing with the timer settings.