I've had a few cordless trimmers over the past 15 years or so. My wife likes using a trimmer and she won't use a gas model. Fine by me!
The first one was a Ryobi 12 volt. It worked ok but the battery was built in, so when it went dead (about halfway through the job), I had to plug it back in and wait. DON'T BUY ONE THAT HAS A BUILT IN BATTERY. After about 2 or 3 years, it wouldn't hold much of a charge anymore.
The second one was a Black & Decker. It was lighter and worked ok but it also had a built in battery. The bearings wore out pretty quickly. I won't buy another B&D. Not very good quality.
The third is the one I've been using for about 4 years. It's an off brand called American Gardener and it works great. It's very light, 18 volt and has a removable battery that recharges in 1/2 hour. I got a spare battery, so when it runs out of juice halfway through the job, I can just plug in a fresh battery. Great feature, BUT the batteries are getting old and they don't have them at the store anymore.
Bottom line - If you're going to get a cordless trimmer, be sure it has a removable battery and you can get spares.
I love rechargeable cordless stuff. I also have a cordless leaf blower (***** for cleaning up leaves but great for drying a car or motorcycle), a cordless hedge trimmer (works great) and a cordless lawnmower that also works great but my lot is only 90' x 100' and only about 50% grass.