I got one when I got my phone on launch day, they were out of stock in the store but I received it on Friday. It's been a nice addition so far, I like the simplicity of quickly swapping one in and out (and as someone coming from the Pre - not having to wait 6+ minutes for a reboot means battery changes are a breeze).
I'm very much a realist when it comes to smartphone battery life and use so until seido comes out with a mammoth battery (which I still may avoid as with case this phone is large enough) it offers a good solution to stay powered up on the go.
I bought this and the dock along with my Epic. I love the "spare battery charger" more than the official dock -- the charger doubles as a travel dock; I didn't realize this at first (and thought the groove was the wrong size), but someone at xda told me if you open the battery door, you can even use it in portrait -- and it works great. Two adjustable angles.
The only minor niggling issue with it being my favorite accessory was when I realized the spare battery charger/dock was the *same size on its own* as my last phone, the Pixi. <_< Wow.