You can also adjust the sensitivity of the tweak, which gives you the option to blow lighter or heavier to unlock/wake your device. The tweak is not always 100% accurate since ambient noises could impact the tweak's detection. Adjusting the sensitivity could help those in noisy environments.
We'd imagine this tweak does have an impact on battery life since it constantly monitors the microphone; however, we haven't used it long enough to know for sure. BlowToUnlock also works with headphones that have a microphone and supports all iOS devices on iOS 7.
You can download it from the BigBoss Cydia Repository for free.