Fast, Light, Compact... What more could you want?
After using my Jetboil for a year now, I can say that it is the best all around cooking system I have used in a long time. While it is not the lightest or most compact, it is pretty much the fastest cooking system out there. A pound is a tad heavy for some ultralighters, but I will gladly make the trade off for speed and ease of use.
The thing that really gets me is the efficiency. The 100 gram canisters made by Jetboil fit right inside the pot and look to be too small to really cook anything worthwhile. So you boil you coffee, the water for dehydrated dinner and so on and it keeps going. It always seems like there is one more left in the tank. With little to no wind, I've had a little over an hour straight out of one canister. That should be enough for 30 cups of coffee, or around 15-20 dinners.
The big drawback is the flame settings. There are only two: off and jet engine. I have managed to cook something as delicate as rice, and even melted chocolate, but it required constant attention and babying. I have learned my lesson and on trip where there is real food, I bring my Whisperlite. Stability is also a little bit of an issue as the pot is pretty tall and top heavy, especially with a full liter of water, but since the Flash comes with the stabilizer, this is less of an issue.
To wrap up (sorry, I like to give long winded reviews) if you drink coffee/tea, or generally need to boil water outside quickly and efficiently, get a Jetboil.