It depends on what you're doing. Most of the time, the fans are totally inaudible. Even at full bore, it's an unobtrusive "woosh" not an annoying "whine." And Apple probably has the best fan control in the business in terms of not fluctuating between different speed ranges. It's actually kind of a shock to go back to a ThinkPad and hear the fan struggling.
The only time I really notice the fans is when I'm running a website in debug mode in visual studio. Note I have no issues with running a vm in VMware Fusion all day, or with using visual studio in it. Which I assume is a fairly high base level load.
I'm thinking of getting one, but the people I know who have them tend to use them in noisy environments and can't answer my questions.