Time Capsule alternatives
Since there have been no "hardware" answers by someone who has used one, I'll add that there are third party drives who support Time Machine.  Just as some examples, LaCie has support in their NAS products and Drobos also have support.  I've used neither, although I have a LaCie d2 Network 2 on order.

The cautionary note here is that "support" means that they (a) make it possible to connect to the NAS from Time Machine and (b) typically provide size restrictions on Time Machine so that it doesn't eventually consume all free space on the drive.  Apple does not support any NAS other than a Time Capsule, so there is no guarantee that the third party drives won't stop working with a future Time Machine software update, or have other problems.