The Companion 5 attracted my attention for two reasons. First, it works via a USB connection to your computer, bypassing the onboard audio circuitry and employing its own. That cuts out system noise and interference. It means generally cleaner sound. If you want to know what I mean, plug a good pair of headphones into your computer's audio output jack and listen to the static and pop.
Second, the 5, which has the standard two speakers and a subwoofer configuration, simulates surround sound that typically requires five speakers. It is not perfect — I wasn't fooled into believing that I had a hidden set of speakers behind me — but it definitely enhances the soundtrack of DVDs played on your computer. Plus it is self-evidently easier to do speaker placement and wiring with fewer speakers.
Both the QuietComfort 3 and the Companion 5 attest to Bose's strongest audio suit. Nobody gets more sound out of a smaller package than Bose. And nobody does a better job of finding way to work around less-than-optimum acoustic conditions. Let's face it. The cabin of an airplane or your office desk are unlikely to offer concert hall ambiance.
On the other hand, they also demonstrate the downside of Bose. Bose tends to put acoustic magic ahead of musical accuracy. With all that surround emulation going on with the Companion 5, there is not as much clarity and frequency response as I would like. The QC3s are even more problematic; to compensate for the lack of passive sound reduction that earmuff phones provide, Bose increased its reliance on noise reduction circuitry. Initially I was impressed by the silence they produce, but after extending listening I grew tired of the obvious distortion to the music.
In addition, they are expensive: $350 for the QuietComfort 3, plus another $40 for cellphone kit; $400 for the Companion 5. There are less costly alternatives, even from Bose. QuietComfort 2s, in my opinion, have a much better balanced sound than the 3s and cost $50 less, and also have a cellphone kit available. The Companion 3 at $250 foregoes the surround effect, but is definitely a speaker system to consider if you are short on space at your desk.