I am a c:geo user and love the app! It's what got me into Geocaching! It does, however, chew through my cell battery. My wife bought me a Brunton Restore Portable Power Device for my Birthday and it has been great! I haven't really used the solar panel functionality but a fully charged device adds several hours to my Geocaching adventures.
http://www.rei.com/product/800499/brunton-restore-portable-power-device
I still find the biggest, and most obvious, drawback to Geocaching with c:geo is when you are out on the hunt and leave cell phone range. You have to know your terrain and be prepared with a GPSr for backup. Even so, I wouldn't leave home without my c:geo!