HCI specialists have metaphors they like to use to describe how products can be made more usable. One of these metaphors is the toaster. They may explain how a complex copy machine, or a PC printer, should behave more like a toaster. You should be able to walk up to one you've never seen before, be able to tell what goes in what hole, press a lever, and reasonable default behavior should pop out a minute later.
Now this is fabulous Crossing the Chasm advice. But the early adopters don't buy toasters -- they buy electrical thermal-transfer bread processing systems. They are interested in the options in the print dialog box -- or to use a literal example, my friend's ancient toaster (an early adopter toaster, as it were) had a tuning screw on the bottom which could be adjusted to calibrate the toaster such that the color of the toasted bread exactly matched the color on the enamel toast darkness selector.
So what's my point?
I recently got to try a Roomba home vacuuming robot. The robot, an interesting piece of technology from a company that clearly makes some hard-core gear, came with about a 4-page instruction booklet. The instructions try to follow a toaster model: simple, clean, "nothing complicated here" -- push "Clean" and go. But it's not a toaster, it's a domestic robot! Like in the Jetsons! This isn't like buying a lamp (and it's not cheap either), so I want to know exactly what the heck it does. It's probably best if I don't have to read a 50-page manual to use it, but I would certainly appreciate one that answers some basic questions.
I'm the guy who's going to be recommending (or not) this device to the people on the other side of the chasm. So when it fails to manage its battery charge and fails to return to its charging station like it's advertised to, I'm the guy who will be very sympathetic if only I can get some understanding of what it's trying to do and why it fails.
A device with great docs is a Samsung VCR-style DVD recorder which I bought a year or two ago, when those devices were a little more raw. It's a solid piece of equipment, and easy to use without reading the manual. But it came with 50+ pages of docs so that I could understand exactly what it could and would do, and where some less familiar recording formats (like DVD-VR) came into play. Because I understood what it could and couldn't do, I felt comfortable recommending it to less technically inclined people. Those people in turn don't want the 50 pages -- they want the toaster, and good for them!
As for iRobot, they score massive points for putting a serial port on the outside of the device and encouraging people to reprogram it if they want! I love seeing a company that isn't afraid to say, "Hey! You bought it, it's yours, go ahead and hack! We're mature enough not to worry about managing perceptions or fighting imaginary lawsuits if you make it do something stupid." That alone is enough to make me forgive if it mysteriously misses a swath of carpet.