I took my new MacBook Pro on a business trip for the first time last week and I wish I had this extra battery. It's true, you can squeeze about 3+ hours out of one battery but I'm spoiled and not about to set my LCD settings at 10% brightness. Getting a second battery doesn't just give you another 3 hours, it give you peace of mind (especially when your connecting flight just got delayed).
My battery in my macbook pro 1.85 ghz lasts up to six hours. normal use gets just under 4 and 1/2. Considering that most batteries on the market last up to 3 hrs, this is great. Also the mbp is exponentially faster than its older cousin the powerbook g4, the fact that it has all of the newer features, runs for the same price, and has longer battery life, this is a great deal too!