Too bad you missed the Pogoplug a couple of months ago at Best Buy if you are interested. I think they were being introduced at that time and there was a great introductory price on them for a week... $50! Compared to the usual retail that they are selling for at the present time, $100. I picked 2 of these units up at the $50 price. One for myself and one for my father to try out.
I haven't used an Airport Extreme with an external HD attached to it. So I can't compare it to that. However, I think the Pogoplug does much more. Basically, you can attach up to 4 USB drives (hard drives or flash drives, both work) and these USB drives can then be accessed by you within your local network as well as from anywhere in the world via the Internet. Only you can access the contents (it takes a log in/password). You can access the Pogoplug with a Mac/PC/Blackberry/iPad/iPhone. The one thing it has that I don't think the Airport Extreme/Airdisk combination has is the ability to share content with people. I will describe that below.
Let's say I have a file or several files that are quite big. Too big to send as an email attachment. But if it is on my Pogoplug attached drive, I can specify the file(s) I want to share with someone by sending them a Pogoplug invitation. There is no file size limit in this case. The recipient of this invitation can then download the file directly from my Pogoplug attached drive. My computer does not have to be on since the drive is attached to the Pogoplug which in turn is attached to the internet via my router. Basically, I can run my own little file server with this simple set up. I can share as much or as little as I want with other people.
BTW, Pogoplug was one the winners of Macworld's Editor Choice Award for this year: 26th Annual Editors? Choice Awards: the complete list | Topics | Macworld