We've been using Disneyland Express almost every trip for the last 20 years. They have definitely changed details frequently over the years! The latest time we used them was for a trip this year end of September/beginning of October. They pick up from the airport every hour, stopping at all the terminals (they are often running late, but not always). I can't remember the time of the last bus though, but it is much earlier than it used to be! Vouchers used to be for a specific date, they are now good for one year from date of purchase.
For pick-up back to the airport, if you are staying at one of the Disney hotels they pick up every hour, and they now have separate schedules for LAX and SNA (instead of having to swap buses at the DLH, which was a pain). If you're at one of the Disney hotels, you don't have to call to schedule a pick-up. If you're not staying at a Disney hotel you WILL have to call to schedule a pick-up, and they pick up every two hours at non-Disney hotels. It used to depend which hotel you were at, but I believe you now have to call and schedule a pick up for all non-Disney hotels. In the last five years I'm not sure that I can remember a time that they haven't been late picking up from our hotel to go to the airport (Disney hotel or otherwise), so I agree with the PP who cancelled for the taxi, I definitely wouldn't cut your pick up time too close!
It certainly isn't the cheapest or fastest method for transport, but we've always found the drivers to be pleasant, some are downright hilarious, and we've always felt the drivers are safe on the roads. Plus, who doesn't want to start their trip with a happy Disney-wrapped bus?
