Just a warning! There are these white 15 passenger van drivers that have a very similar name that stop at the same stop as the REAL Disneyland Express. If they don't see you with a voucher, they will try to tell you that they are the real thing and they are NOT. They might be a bit cheaper, but I wouldn't trust them!
Be safe; wait for the big bus with wrapped in Disney images. You don't have to pay ahead of time - they make a stop first at the Disneyland Hotel and take vouchers or payment for tickets.
I believe they run every hour early in the morning, then every half hour through early evening, and back to every hour at night.
It's a great way to get there and they stop at practically all the "close" hotels. Sometimes one stop may be for multiple hotels with them being so close together, so keep that in mind that you may have to walk a bit to get to your hotel from the stop but never very far.
We have taken the DL express both times we've been to DLR and never rent a car. We love it!