If you are staying onsite at one of the three Universal hotels, Unlimited Express access is included in your room rate.
From what I've read, the price of the pass fluctuates based on the anticipated crowd levels - the more people they expect in the park, the more expensive it is to buy an express pass (because there will be more demand for them). For example, the week I'm going in October, they must be expecting relatively low crowds, the express passes are only $25/per day for both parks together. My assumption is, the more expensive the Express passes are, the more likely you'll need them.
You might want to think about which rides/attractions are most important to you, and be sure those things participate in Express before purchasing. Not all of the rides and shows have an Express line. Some rides also have a single rider line, which can shorten your wait if you are willing to split up your group. ETA: Harry Potter and the Forbidden Journey and Ollivanders Wand Shop do not have an Express line.
You also might consider how many days you'll have at the park and how long you're willing to wait in line. If you have a few days and do not mind waiting, maybe the Express pass is not necessary. If you are only going on one day, and want to be absolutely sure you can do everything, you might want to go ahead and purchase the pass.
You can check the crowd levels thread to see what others have posted about those dates last year. Good luck!