We get it by booking a deluxe onsite hotel. It comes with such stays at no extra cost. You get two days' worth of it for a one night stay.
It's immensely worth it IMO, and it's hard for me to imagine Universal without it (it's impossible for DH and DS to imagine it...I wanted to try out Cabana Bay this trip and they absolutely refused to consider it since EP doesn't come with that resort). Many others have gone at less busy times and have been fine without it; we love it even if it turns a 10 minute line into a 5 minute line.
It works on Flight of the Hippogriff and Dragon Challenge (the two proper HP coasters). It doesn't work on Gringott's, Forbidden Journey, Hogwarts Express, or the Ollivander's Wand Shop show. There's one new ride in the Simpsons area that it doesn't work on, or so I've read. Other than that, it works on everything else. Which is awesome.
If you're considering buying it, depending on the size of your family, you might look at an onsite deluxe stay, just in case. I use this site to book:
http://uo.loewshotels.com/en/booking/uo?hotel=UEPBH,UEHRH,UERPR Depending on when you're going, if you're doing one or two days, what you want, and how many of you there are, it *could* turn out to be cheaper to rent an onsite deluxe room and get the unlimited Express Pass included, rather than buying it for your party for a day or so.