Definitely worth staying onsite for the free Express. As mentioned above, you can always buy Express if you don't stay onsite, but at that time of year, with Express running $50+ per person, it just makes more sense to stay onsite.
Now, Express is supposed to guarantee no more than a 15 minute wait, even on busy days. The reality is that the parks have a tendency to over-sell the paid Express passes, meaning that the Express lanes can get clogged. However, the clogged Express lines can usually be avoided through use of common sense. For example, Earthquake is located right next to the Beetlejuice theater. Therefore, when the Beetlejuice show ends, it seems logical to go on to Earthquake. That's great in theory, except 1500 of your closest friends have the same idea. Earthquake only holds 220 people per show (and it's the largest capacity in either park, other than show venues). So do the math, and don't go to Earthquake right after Beetlejuice ends. It's similar at all rides. Don't go on a ride that's located next door to the show you just left. Don't get in an Express line that looks visibly backed up. Backups can happen for many reasons, especially if the ride was "down" earlier in the day.
This may sound like obvious advice, especially to those who have been to Disney. But the key difference is that Disney's FastPass machines will only give out x number of FastPasses for any given time slot. Universal's Express lets you go to any attraction at any time of day, so to maximize your experience don't go at the same time everybody else does. If the Express line looks long, it probably is. Go on to another attraction. Then come back in an hour or two and the original line will be down.
Have a terrific time on your trip!!! Feel free to ask as many questions as you like!