Why jQuery? Unlike some of the other libraries, jQuery is very lightweight and does not suffer from a lot of bloat. You can also find a number of well documented plugins to do just about anything. You also have a great deal of UI controls and widgets that are available by using jQuery UI. Did I mention the documentation – it is really great. The learning curve is not steep – and once you do a few smaller projects – your comfort level with jQuery will skyrocket.
So next time you decide to do something to add some extra pizazz into your web site – remember, take a look at jQuery first!