I maintain three different iTunes libraries. The one on my Mac Pro is for audiobooks and podcasts that I keep synched with one of my iPhones. The one on my MacBook is for video podcasts and television/movies that I would watch when the screen is close to me, or plugged into a large display with HDMI. Lastly, I keep an iTunes library on an external bus-powered USB device. I take this drive with me everywhere and plug it in whenever I want some tunes. It's got about 150 Gb of complete albums with proper metadata.

One of the frustrations I encountered was switching between iTunes libraries. It's easy enough to launch iTunes while holding down the Option key, and then navigate to the appropriate file. I think a pretty good solution is to create aliases to each library in ~/Music/ because that's the path that is displayed in the file selection dialog window. Picking the right one just requires a double-click on the icon, rather than navigating to ~/Music/iTunes/ or /Volumes/PortableDrive/Music/iTunes/.

Here are the two alias files I created:

This is the first window that is shown when starting iTunes with the Option key down:

Then I select the iTunes Library file I want, et viola!

I hope that was useful.

PS: Yes, I know that's not how you spell "voila."

PPS: Windows user can hold down shift when opening iTunes. However, if you use Windows, iTunes is the least of your problems.