Each of my last three trips, I've gone from Disney (Wilderness Lodge) over to Universal/IOA for a day, and I can tell you it's really quite simple. I relied on the folks at Mears, and it was great. I called the day before, and they asked what hotel I was staying at and let me know about what time they'd be there. Both times, they arrived within about 15 minutes of the scheduled time. We then went to a couple other resorts, picked up guests, and tracked on over to Universal.
As I approached the end of the day, I called Mears again, telling them, "Take me home!" and they gave me a time and a gate number to be at. It was really very, very simple, comfortable and inexpensive. In fact, I'm planning to do it again during my Disney trip this year (nov. 29-Dec. 6).
Hope this helps!