Its only drawback is being absent if you have to disable addons for any reason, such as troubleshooting or expansion start. It greatly increases the 'space' available on your buttons - for example, I have a right-sidebar button on my Druids just for quick spec change macros (changes spec, gear, title and transform in one go).
I have flyouts for battle pets appropriate to my toons, and sets of mounts (ground, flying, swimming, utility). You can move the side off which the flyout extends easily with a right-click. It's a compact solution to the valuable-screen-real-estate issue.
Now, Otenth might have shown me this one back when, but I can't recall - I'm pretty sure it was recommended to me: Flyout Button Custom
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