I am going to buy this add-on. You don't have to ONLY use this mode of transportation, it just gives you another option. The beauty is that you know when it will leave, if it works out that you are near the Express Transportation kiosk at a good time and you want to hop to another park, then hop on - if not, then don't. I think its going to be a huge timesaver for us - we'll be able to get from the middle of one park to the middle of another without the hassle of the main entrance/security/long walk to the buses, etc..
For $29/week its silly not to get it (if you plan on park hopping). That's my opinion, obviously!