<Too bad Guinness in a can is sooo much better than Guinness in a bottle.>
It's important to note that the "Rocket Widget" is intended to allow you to drink Guinness straight from the bottle. If you pour it from the bottle to a glass, you'll lose the benefit of the widget releasing nitrogen gas with each tilt of the bottle. You'll notice a definite difference in taste if you compare glasses poured from the bottle vs. the can. If you want to drink from a glass, buy the can (or have one poured from the tap).
Edit:
Oh Yeah -- How many calories are in Guinness?
10 calories per ounce. Less than skim milk!