the benefit of synergy is being able to control an application, like itunes, without bringing its windows to the front. With something like Safari, however, it is already the front application when you're using it, and you really wouldn't benefit from being able to control it while it's in the background.
In an ergonomic sense, there's no benefit in separating the controls from what they are controlling in this instance. What would be helpful, however, would be adding basic browser controls, like back, forward, stop and refresh, to the contextual menus, allowing a right-click (or command click) to summon the commands wherever your cursor may be.
But, yes, for iTunes, Synergy is quite good, and pretty darn slick, too. I expect it, or something like it, will be integrated in future version of itunes.
ciao,