I don't have a sonic toothbrush, but it is a electric, a Braun Oral-B ProfessionalCare 8850 DLX and what I like about it is that no matter how tired I am (or not awake) it makes brushing easier so I always do it. I don't know if it's because of using the electric toothbrush or just because I'm more diligent because of it, but, my checkups are better and my teeth feel cleaner.
I use a Soniccare and feel the same. Part of it is you are guaranteed to get the recommended 2 minutes of brushing.