When I got my 21.5" mid-2011 iMac with extended (wired) keyboard and Magic Mouse, I found that the NiCad AAs that came with the MM lasted ca 3 weeks — not an auspicious sign.
I decided to get Apple's Battery Charger (MC500LL/A, Model A1360) which came with 6 rechargeable AAs (NiMH) — and at the time Best Buy had an even better deal than the Apple Store. I've been more than satisfied with their performance.
Since January 2012 I've recharged the batteries (in rotation) 57 times and my average use is 3 weeks (mean 21.3 days, range 16-32 days). The batteries still function as they did when new.
Take that for what it's worth.
