I see two main categories for the floating menus, ones powered by Javascript (typical) and ones powered by CSS (not compatible with older IE). The JS kind I don't like because it's distracting because it always has to reposition itself as I scroll the page and I've left pages just because of it. The CSS ones don't have jerky movement (at least ones I've built don't) so they are fine with me. There can be some accessibility issues with it though for some screen readers since they're usually done by setting position:absolute, and those elements don't get read by the screen reader (not all screen readers behave this way).
That's my 2¢.