Opt for a rechargeable model
I've had a Sonicare toothbrush for about 15 years now and I love it.  On her dentist's recommendation, my fiance wanted to give one a try.  We got her the battery operated model since she was unsure she'd like it.  She LOVES it though.  The one downside to it - and it's a big one - is that the battery life leaves a lot to be desired.  She's been using it about six months and we've replaced the batteries about six times.  Had I known the battery life was so abysmal, we would have gotten her a rechargeable model.
Rating: 2 stars