I just had an Interlock Kit
http://www.interlockkit.com/index.htm installed by my electrician. It passed local code and is UL approved. See attached photo (not my set up). He also put in an outside connection box which feeds the generator into the circuit panel. It is designed so that you cannot turn on the breaker for the generator to power the panel unless the main breaker is off. Like others here, I had a portable generator that I used primarily during that big ice storm a few years ago and during another lengthy local power outage last year. I tired of running extension cords, dealing with getting power to the furnace, etc. I looked at both the traditional transfer switch with 6 to 10 circuits which must be chosen in advance and once chosen cannot be easily changed and the interlock kit. I went with the interlock kit because it supplies power to the entire circuit breaker panel. Obviously, you need to know how much you can power at once, but the interlock does give you the flexibility of, say, powering off most everything to run the microwave, oven, dryer or other high watt items if your generator can handle it. Now I am thinking of getting a bigger generator. Marking the essential circuits, the other circuits one might like and the circuits which should not be used unless most everything else is off (dryer, microwave, oven, etc.) makes it pretty easy. I run two refrigerators, two sump pumps, oil furnace and 5 or 6 lights all at once with no problem. A week after it was installed, we lost power for 5 hours and it was a breeze compared with the extension cord route. I turned on all circuits except the dryer and the stove and was careful not to turn on all the lights in the house.
To answer the question about the 2008 ice storm, in the southern area of the state, people were without power for anywhere from 2 to 3 days to upwards of a week or more. In the lakes region, my house was without power only for 4 to 5 hours (thank god for Freeze Alarms!!). Go figure, but the ice was much worse in the southern and western parts of the state than further to the north.