This is a really useful command from Tips4Linux.

I typed this

grep -F capacity: /proc/acpi/battery/BAT0/info

on my EeePC 901 and got this output:

design capacity: 6580 mAh
last full capacity: 6461 mAh

So, still more than 98% of design capacity after 6 months of constant use.

On my 18 month old MacBook, I got

design capacity: 50200 mWh
last full capacity: 46930 mWh

which is a bit more than 93%.