We used it when we were there May 1-8. Other than it shutting down if there's a lightning storm, it was great!
In each park, a bus to another park runs every 30 minutes, typically. And it will be fairly fixed throughout that day. For example, if you're in Epcot, the bus to MK may run on :00 and :30, bus to DHS on :10 and :40 and to DAK on :20 and :50.
When you first get to the park, swing by the Express Bus stop and ask them what the times are for the park you think you'll want to head to later, so you know for sure when to be back.
You have to check in at the little table at least 10 minutes before the bus departs. They scan your MB to make sure you purchased the pass, and that you have a Park Hopper. Then you can sit and wait, go to the bathroom, or wander nearby until about 5 minutes before departure.
They'll have everyone heading to that specific park queue up, then a Guest Relations CM will escort the group through the backstage gate and to the bus.
I don't think we ever rode a bus with more than maybe 3-4 other people at any time.
If they continue to offer it, we'll always do it. Bypass security and a ton of walking.