Help my mother select an electric-assist bicycle!

So my mother, the 45-year-old elementary school teacher, has decided that in keeping with her image as a yuppie granola-crunching Oregonian, she should ride her bike to school more often. Now, she lives maybe five miles away from the school on what used to be a nice, quiet country road, but is now a single-lane, moderate traffic highway, with traffic usually at around 55 miles per hour. That’s not a problem, really, the shoulder is plenty big enough for bicyclists, but it’s there for background information.

This has been great, actually, for the last few years, but she’s getting a little more serious about training for her marathons and doesn’t always want to ride her bike both to and from work some days because she’s too tired for it. In addition, she’s kind of limited to weather in which one wants to be outside riding a bike for a good half hour or more, and if she can cut down the travel time a little, she’ll be able to continue into the colder months a little better. She’s also looking to be able to go shopping after work without exhausting herself.

So she’s looking for an electric-assist bike, where she can get just a little bit of boost for hills and the like so that she can go just a little faster and further. What experiences have you-all had with these? How do they hold up in bad weather? How far can they go on a charge? What’s the battery life (I mean long term, after many recharges) like? What should I avoid?

Thanks in advance for all the help!