I've recently upgraded my Etrex 10 to the Garmin GPS64, with full UK OS 1:50 maps.
So far very impressed. Used it to follow a sketchy path out in the dark and it appeared showed we were on track despite no path, but zoomed in and it showed we were off it, no surprise, and was able to find it again, spot on. Many other features, yet to be tried, but we'll impressed
Like the previous post I still like a paper map for that bigger picture, but so far I can't fault the 64 for accuracy down to a few metres.
Not sure on battery life, as I never kept my previous Etrex permanently on, only using it when needed.
I'm looking to get a set of rechargeable ones, Eneloop, came highly recommended over the 64 battery pack, but if I use it as per the old one, can't see an issue, and I always carry spares.