As all other features, it's a good thing when applied judiciously.
It is being used for the tooltips of all applications, so widely that it has become a known UI idiom. For web applications, the browsers wait until you park the mouse pointer for a short while before tooltipping you. Desktop applications also do so.
The jQuery hoverIntent plugin mimics that behavior for all other hovering effects in web UIs.
It solves a nasty issue with menus. See this example: imagine you want to print this newspaper page. Your mouse pointer is at the center of the top-most menu bar, over Susana. You move it straight downwards aiming at the printer icon and click: you end up looking at the obituaries page ("Fúnebres") because you briefly hovered over "EDICIÓN IMPRESA" in the light blue menu bar and opened a choices list that covered your target.
Should the blue menu bar had hoverintent implemented, this wouldn't have happened.
Notice that this is especially annoying for the more expert users, who would be mousing swiftly.
The plugin has a parameter, a milliseconds timeout, the time the user has to hover the target in order for the corresponding action to be fired.
If you choose a low delay the UI behaves more like normal, the delay isn't noticeable but the user would be reaping the plugin benefits without noticing it, which is BTW how good usability has to be.