After years of using a whisperlight, then getting the internationale, I finally upgraded to a jetboil.
The MSR's were great, so good that both of them disappeared into other people's kits at some point or another. I have to echo the other people's comments though. The soot was nasty upon priming, and keeping a simmer was really tough. I also found I did not use them all that much and had to constantly replace the o-rings. I also had more than one issue with the pumping thingy.
Also, I pretty much used the windscreen every time, even the slightest breeze and cold temps would increase the boil time substantially.
The other thing I did not like was the complexity. you have to keep the service kit with you and be prepared to use it (o-rings were my bain). Also you have the fuel bottle (with a cap) and the burner unit. Sometimes finding a flat spot is a pain.
As for the JetBoil....I love it. the canisters are easy to find and use. The whole setup is really clean. The only gripe I have is the stability. it is a tip over waiting to happen. There is a reason I never order a cocktail in a hi-ball glass........Shoot I am even skittish around stemware...
The other plus with the JetBoil, I have a Optimus Lantern that uses the same canisters, both units plus a couple of canisters barely take up any space in my kit.
The Grand Caveat for all this follows: I do a lot of cooking over coals, or in a Dutch oven, so the stoves primary duty is for boiling water. A buddy has a single cartidge burner that we use to simmer sauces, stir fry etc... It is actually a pretty cool unit, he picked it up at an asian food market for $20, it is meant for the table top. I have yet to field test (or home test) cooking an actual meal in the Jetboil. Some of the recepies on the site look pretty tasty though.