When I worked at Universal in 2012, we were told to tell guests that the Express Pass cuts your wait in half. So if there is an hour long line, the Express Pass will get you through in 30 minutes. Not sure if that is still true or not.
There are pros and cons when comparing Disney's Fastpass and Universal's Express Pass, and one pro that Disney has is that, for the most part, the Fastpass system only gives away so many Fastpasses at a time. I have no idea what the numbers are, but just to throw around a number let's say Splash Mountain only has 1000 Fastpass reservations between 9:00-10:00am. Universal's system doesn't keep track of that, so theoretically everyone with an Express Pass might decide to go on Hulk at 9am which greatly increases that Express line, while no one is using it for Cat In The Hat.
At the end of the day though, it will be similar enough to Disney's Fastpass. Even with Fastpass, there are times you still have a line. I've definitely had to wait a few minutes on some lines. It will be the same with Express Pass. However, the Express Pass line will always be shorter than the regular line, so no matter what you will be saving some time. And saving even a little bit of time is nice.
And, just as a note, remember that some attractions will always have waits unless you time things just right. For example, when I worked at Universal I worked on Shrek 4-D, which is a 3D movie type attraction like It's Tough To Be A Bug at Animal Kingdom. Nearly every day someone with an Express Pass would complain about waiting on a line to get into the theater. The problem was that you are entering a theater! The show starts and stop and sometimes you have to wait 10-15 minutes for the theater to empty before you can get on! I tried to explain that it's not like the Haunted Mansion where the line is always moving because the ride vehicles are always moving. But people expected with the Express Pass that they will never have to wait on a line, but sometimes it's just the nature of how the attraction operates. I actually knew someone who worked on It's Tough To Be A Bug and they got the same complaint from guests!