We booked a trip with Belize Shuttles from the airport to San Ignacio. Everything was on time and just fine. Well, our American Airlines flight was really late, so actually the driver had to wait for us, and that probably made him late for his next pickup. But he didn't drive like a maniac to make up for it -- it was all safe & above board.
Our shuttle from San Ignacio to the Belize City ferry dock was over 6 hours late, so when it looked like we would miss the last ferry for the day and couldn't wait any longer, we grabbed a cab. The driver was great, the cost was similar, we made the last ferry, and everything worked out fine. He was a local from Santa Elena, and had once lived in Belize City, so we learned a lot from him!
The thing with the shuttles is that Belize is such a small market (compared to a high-volume market like Cancun), and the destinations are relatively far apart, that it's not economical (or ecologically sound) to drive an empty van 2 hours into the jungle, pick up 2 people, drive them to the airport, pick up 2 more people, and then drive the empty van back to the airport. They have to consolidate trips to make the business work, and sometimes that results in delays. An even bigger impact comes from the fact that they are kind of at the mercy of the airlines' ability to arrive on time.
So I think they are making reasonable business decisions given the circumstances, but you have to be OK with "Island Time" in order for this to work for you. And of course, this was just MY experience.
That said, I heard great things about William, but he wasn't available when I needed to travel. :-) And the two folks who have already replied to you -- hopefulist and Dr.FeelAwesome -- gave spot-on advice, so you may want to check out some of their other posts while you're planning your trip.