What are the advantages of Time Capsule over Airport Extreme with an external hard drive? I would like for someone to outline the differences between buying a time capsule vs. just getting an airport extreme and attaching an external harddrive. The latter seems more flexible (easy to change external harddrive) and even cheaper, yet people are getting time capsules.

What are the pros and cons.

Thanks.

UPDATE

Could you please elaborate on:


How easy is it to setup with an airport extreme. (Apple explicitly mentions you can add a harddrive to airport extreme, but doesn't say anything about time machine. Is there anything more to it?)
What are my options if the Time Capsule harddrive fails and/or I need more storage later.
What are the differences in the router capabilities, e.g. - number of ports, number of supported users, usb jacks, etc.
As above the Time Capsule is the easy solution.

I can only really comment on your second point, I bought one about 3 years ago and 18 months ago the hard drive failed, it seemed to be a known problem as Apple replaced the Time Capsule free of charge even though it was outside of the warranty period. What could not be replaced however was the 18 months of backups as Apple would not have replaced the Time Capsule had I opened it up to get a the drive and try to recover the data. So after that experience I won't be buying another Time Capsule.

If you are not particularly after a router there are now other companies that offer Time Machine compatible network storage, which may be worth looking at.