I have used mears several times to go between Disney and Universal, and found it to be efficient. As Goofy4Tink says, call the day before to arrange for your pick up. They usual send a van, and may pick up at other resorts (either on or off property) along the way. One time I was the only person picked up, and they sent me in a cab. You also may have to make a stop to drop off at Sea World along the way. I'm usually picked up around 8:30, and I'm in the park within 5 minutes of the opening time.
You can either arrange your pick up time in advance, or call about 2-3 hours before you want to be picked up for your return trip. The return trip is usually in the bus, and will make other stops along the way. I would plan for it to take between an hour and 90 minutes to get back to Disney depending on where you have to stop along the way, and this may also include a pick up at Sea World.
I can't remember how much I paid the last time I used Mears, but it was under $20.
If you have more than 4 people, you may want to look into a town car or a van. It may be cheaper than mears, and would cut out the extra stops.