The battery of my Forerunner 205 was failing: the unit would have the low battery message on at about the 2 mile (running) point.
The replacement battery arrived promptly and was reasonabley easy to fit, but not just a simple plug in. The case has to be cut open with a craft knife, the wires cut, re-soldered and sealed with shrink tube, then the unit glued back together (I used super glue). None of this was very difficult and the unit charged up straight away afterwards I suppose the repair took about half an hour.
My Forerunner should be good for another 5 years now.