I own both the 640 and 640 XL and I would guesstimate at least 60% of the time the double tap to wake doesn't work on the first try. Sometimes it takes three or for attempts. What about you guys and gals?
The double tap works too efficiently for me, it wakes up in my pocket sometimes, it's so annoying I don't even use it anymore
I had a 520 and I now have a 640XL rule of thumb so to speak on double tap to wake just double tap on the bottom half of the screen and you should have 100% success. As the above poster said don't have your hand/fingers shadowing or covering the sensor.