The device itself resembles a slightly obese thumb drive. Elgato recommends plugging it directly into the computer, which may present a slight problem for owners of iMacs that have just three USB ports.
The first port probably takes care of your mouse and keyboard (unless you're using wireless peripherals), and you may be using that one with an unpowered hub to allow easy access to thumb drives.
You're unlikely to be interested in the Turbo.264 HD unless you also own a TV tuner, and our experience is that they generally work better when connected directly to a computer and not via a hub. And the main point of using a Turbo.264 HD is to prepare content for an iPhone or similar device, and again, direct connection seems preferable to an unpowered hub.
So you may need to unplug either the tuner or the iPod/iPhone cable to use the Turbo.264 HD. Since current iMacs have four USB ports, so that's another excuse for an upgrade!
What about the software? See page 2.