It is similar.Airport Extreme + USB drive is functionally identical to Time Capsule, kind of?
I bought Airport Extreme time ago, but desperately failed to configure it to join wifi network (which it actually should be able to, when using older Airport Utility). It is sitting idle now.
I'm just not sure how robust it is. I later disconnected those drives and turned them off. Any time I tried lengthy copying or copying of more than a few hundred MBs the drives dropped out. Then it became a series of turn the drive off, then back on in order to get it to show back up inside the Airport utility.
That could be down to my cases though (I doubt it's the hard drives inside) as their functionality has seemed to degrade over time.