I have had my Pure ONE radio for nearly two months now - its excellent (see my review). I think the chargePAK is a bit on the expensive side (otherwise I'd give it five stars) but, by golly, it does the job.
It slots into the battery bay, a slightly fiddly little plug connects it to the radio and off you go. An initial charge is recommended for 24 hours, and then the radio can be plugged and unplugged from the mains as you like - it automatically moves onto ChargePAK power as necessary, or starts recharging when you plug it back into the mains. I think its great - I usually have the radio plugged into mains in my office where I do most of my listening, but I can just pull out the jack plug and carry the radio into the garden or the kitchen and the sound just follows me around.
What can I say - it works!
UPDATE - Some Time Later
Oh Dear. This did NOT live up to its promise. I discovered after a few months that it didnt hold its charge for more than an hour - If I was lucky. I originally gave this four stars - now I want it to be one or none. Not sure if I can change the stars. Radio is great though!