I had a serotta fit a few weeks ago. For me, the thing was really a learning experience about my cycing in general. This was because the fitter explained why different aspects of the fit mattered to how I rode.
Remember to bring your regular cycling attire including clipless shoes and pedals, and tight fitting clothes (of course I forgot that ***** at first
...) . They give you a pretty extensive questioner and take lots of measurements before you even start on the fit cycle. It probably doesn't need to be said, but be realistic and honest with the questions. BTW, I'm not planning on getting a serotta, but the fit is pretty universal to all bikes. After the fit, they set up all my bikes with the "fit" we came up with. Turns out, I wasn't that far off to begin with. But like I said, the whole thing was worth it for the learning experience.