I have been working since August 2010 almost every day with the mentioned lawnmower.
The mower is used intensively and battery problems I know of nothing to report.
The battery lasts about 300-350m² with me depending on grass height. 3 months ago I had even bought myself the larger battery. It was expensive at 200, but still cheaper as vergliechbare replacement batteries other lawnmower manufacturers.
So I'm more than happy with the device.