The X-Fi HD is something of an oddity. It's a USB-powered external soundcard designed for laptops—and when it comes to delivering awesome audio, it exceeds expectations. However, the features of the card leave you wondering who Creative had in mind when designing it.
The X-Fi HD USB is based on the same architecture as the X-Fi Titanium HD, and you can expect the same kind of sound out of it, including striking Dolby Digital support and THX certification (meaning it passes rigid sound standards set by Lucasfilm Ltd.). You'll have no issues getting lost in deep soundscapes that'll sweep you across Azeroth or have you diving into the trenches of Germany—tweaked just how you like it thanks to great drivers and software like Creative's ALchemy and EAX Studio. Unfortunately, due mainly to the higher noise and interference you get from running power and input signals through the same USB cable, and the lack of ASIO support (which helps prevent lag in sound signals), the overall quality isn't quite that of the Titanium HD.
Oddly lacking on the X-Fi HD are any 3.5mm audio jacks—instead there's a .25 inch jack. This means that with most headsets, you'll need a .25 inch to 3.5mm adapter, which is annoyingly not included in the box. Just as odd: included on the card is a phonograph ground—you know, for all your portable record-playing needs. That said, there are top-notch RCA and optical inputs and outputs, and converting from RCA to 3.5mm is easily done with an included patch cable, so finding a solution that works for your particular setup shouldn't be too difficult.
As portable soundcards go, it's tough to find anything better, especially at $100. However, if you've got a tower instead of a notebook, look to the Titanium HD for better (although a little more expensive) results.