I bought this at a nice discount after Best Buy price-matched the cost to what another retailer was offering. I couldn't believe how easy THAT process was - the sales associate just looked it up on his tablet and applied my discount upon checking me out. Took a couple of minutes is all.
I already had an Airport Extreme, same generation. This device is basically just an Airport Extreme with a hard disk inside, but no you can''t just add a hard disk to the Extreme, as some of the connectors are missing. The router portion of the Time Capsule is identical to the Extreme in terms of specs.
I could have set this tip as my base station, but since I already have the Extreme as the base station, and didn't want to reconfigure my whole network and have to re-authenticate all my devices, but I needed better coverage in the west side of my house, I added this as an extender. Could not have been easier. Went into the Airport utility on my computer after powering the device up (all I did was plug it into an outlet), and the Airport utility found it and asked me if I wanted to set it up as an extender. I clicked "yes" and in a couple minutes the light on it turned green, and I was up and running. My devices logged into it automatically, and it assumed all of the configuration of my existing network with no input from me. You can see which device is attached to which unit (the Time Capsule or Extreme) in the Airport utility.
My modem and Airport Extreme are in a bedroom at the far end of my house. I put this unit on a table in my living room, roughly in the center of my house (it's attractive enough to be fine in an open area where everyone can see it), and now I get 150-170 MB/s (wireless) consistently on my iMac at the other end of the house (up from about 30 or so). It has three Gigabit ports, just like the Extreme, and even on this Time Capsule which is set up as a wireless bridge, I get about 280 MB/sec on its ethernet port (my internet service is 300 MB/sec). Pretty astounding for an extender sitting in the middle of the living room several yards away from the main router.
The hard drive in the Time Capsule is easily accessible in Finder on my Mac, and effortless to back up to with Time Machine, even with this set up as an extender and not as the main router.
Note that you can plug a USB hard drive into either the Airport extreme or Time Capsule, and that USB hard drive behaves and is visible in the exact same way the Time Capsule's internal drive is - only difference is you can format the Time Capsule from the Airport utility, but you can't format a USB drive via the same Airport utility. However, the Time Capsule is expensive, and this might be an alternative for whose who don't want to or can't shell out the money for one of these.
If you have a large house (I don't), or a house with an awkward setup that makes connection speeds difficult in areas (I do), this is a great option to extend your network with the AC protocol and super fast speeds in a seamless, quick way. It is terrific for those not technically strong who don't want to mess with configuration settings, particularly if you have Apple devices. The configuration is all wireless, and you do not need to connect it via ethernet to set up the initial config, as with many routers.