In one of my projects I use JQuery Tools Tooltip, and it works great. http://flowplayer.org/tools/tooltip/index.html
I just noticed, that JQuery UI is releasing in 1.9 a tooltip as well: http://docs.jquery.com/UI/API/1.9/Tooltip
Should I switch to this one?
Only reason would maybe be if the UI version have some performance or other functionality improvements compared to your current solutions.
But as Karpie says. If your current solution works great there is no reason to switch.
It comes down to if you are also using jQuery UI as well in the site, and if you are using theming or other events that could work back and forth.
Also if you are using jQuery UI already for other things, converting would potentially remove one extra JS script from having to load in.
