There's good and bad about the Companion 5, and most of the bad is just the nature of technology today.
The good is that the sound coming from two speakers and an "Acoustimass module" (read, "subwoofer with circuitry") is remarkably close to the sound you would get from 5 speakers and a subwoofer. The Bose system really does know how to 'fake' an acoustic space, and the circuitry somehow manages to figure out how to split out sound ranges: voices sound clear and crisp and seem to come from out of thin air in the dead center of your screen, ancillary sounds move all around the room through the left and right channels, and low-end whump is modulated well so that low notes don't all sound exactly the same. I've read that the lower-end 'Companion' series lack this degree of sophistication, so keep that in mind if you're comparing models.
The bad is that when it comes to computer speaker systems, you really don't have a whole lot of choices. Not many high-end manufacturers (that I have come across) go to the trouble of making a sound system that's geared towards a computer: top-tier quality in multi-speaker systems is left to the home theater crowd. While I'd hoped to see easy-to-find entries from Definitive, Yamaha, or Infinity, the computer scene seemed entirely populated by Creative and Logitech. Oh, and by the way, Bose makes a few choices.
Still, what an excellent choice the Companion 5 has turned out to be! I set this up on a laptop with integrated sound. It's in a corner that has a window behind the desk, a wall to the right, a larger window, and then the left is a wide-open entryway and there's about 15 feet of open space behind the chair. You'd think that would automatically foul the acoustics but somehow the Companion 5 keeps things balanced in this unbalanced environment. It was amazing to have a second person enter the room and not impact the spatial feel of watching a movie in Dolby Digital. Even sitting behind the user, your ears will get clean separation of sound (though not all the positional feel of someone in the front seat). Windows 7 has the ability to tune the acoustics of a room using the speakers and a microphone, but it turns out this is not only unnecessary, it tends to make things worse: all it seemed to do was crank up the right channel and almost completely mute the left.
Which brings us to perhaps the most important thing to know about the Companion 5: like any computer accessory, you need to fiddle with it to get it to work the way you want it to. Bose's manual seems to carry you just as far as getting everything connected (and their support FAQs seem to avoid doing more than regurgitate pages of the manual at you), so here's what you'll want to do to really get the most out of this system.
Regardless of whether you have a sound card or onboard audio, don't bother trying to use "environmental effects" or "audio enhancements" in your software. The Bose speakers don't connect to your sound card at all: rather than a typical speaker audio plug, they connect to your computer solely via a USB cable. This means that Windows and Mac both see the speakers as an "audio device"--as if the Companion 5 was its own sound card. If you see features in the Device Manager (or Control Panel, or audio software) such as "Bass Management", "Room Enhancement", or "Surround Simulation", don't enable or configure them: the Companion 5 takes the incoming signal and does all this for you, and changing the settings in Windows only gets in the way--we discovered the mid and high ranges were wiped out when these features were on and everything sounded muffled and cheap.
In Windows Vista and Windows 7, all of your "audio devices" are visible via the Control Panel ("Hardware and Sound" --> "Audio Devices"). Use this interface to specify the Bose Companion 5 as the default output device on your computer (right-click "Bose USB Audio" and choose "Set as Default Device"). This doesn't mean any speaker or headphone jacks don't work any more: because the Companion 5 is a USB device, removing it will automatically cause Windows to fall back on the next audio device in line. Though I have to say you probably won't ever want to disconnect the Companion 5: the controller knob comes with line in and headphone jacks, which means you can still use a headset whenever you want and get the quality sound processing of the Bose Acoustimass technology.
Now for the final tweaks: whenever you have a software package you use for playback (such as Media Monkey, PowerDVD, etc.), you'll want to pick a favorite song, play it, and tweak the software's equalizer to get the sound the way you like it. This sounds tedious, but it's not that difficult: dragging sliders for a couple of minutes gives you a pretty good feel for what sounds good and what doesn't. And once you save those settings, the player sounds just right, every time. If you're a Windows user, I also recommend you take the time to configure Windows Media Player's equalizer, even if you don't use Windows Media Player or Windows Media Center: something tells me that browser-embedded audio and video will benefit from these tweaks thanks to the Windows Media Player's kudzu-like infestation of the OS.
Physically, the Companion 5 is nice: the speakers are a bit dense, but stylish looking: they sit on sturdy posts and have a small enough profile to make them easy to ignore. The Acoustimass module is pretty heavy, but also fairly compact: about a few inches bigger than the Logitech Z530's control unit. Best of all, the controller for the speakers is a very classy-looking little wheel, about the diameter of a soda can. Volume is smooth and responsive with a light spin of the outer edge of the unit, turning on and muting is as easy as tapping the center, and the controller has two jacks(headphone and line in) for connecting a headset. The only real complaints about the Companion 5's form are:
a) the bass dial for the subwoofer is on the back of the Acoustimass module, which means lowering the bass will involve crawling on the floor; and
b) there is no "Off" switch: the Companion 5 will power itself down after "several minutes of inactivity", but there isn't any simple way to save energy by, say, tapping the controller.
If you love spending a lot of time in front of your computer watching movies, listening to music, or gaming, the Companion 5 makes a crowning touch to the experience. The Acoustimass technology really does manage sound smartly without the need for multiple speakers tucked away around your desk, and the setup is fairly simple (I'll gladly answer questions & offer help in the Comments section here). The documentation and support could have been a lot better, but with a little bit of first-time adjustment, this system will reward your efforts with excellent sound and minimal crowding of your desk.