We just got back from Disney and I also purchased the transportation. I wish I didn't.
We only used it twice. A lot of the time, when we left a park, instead of going straight to another park to park hop, we would go back to the resort, hitting up the other park in the evening, which wasn't the plan when we first bought it. The first time we used it, it was convenient. got from MK, to Epcot, faster than if we monorailed it, due to being Easter week
But, as I said, starting the day at MK in the morning, then being about 90 degrees by 2pm, plans change quickly, as we headed to the resort to swim and cool off, instead of totally exhausting ourselves and our kids.
So in the end, here is my conclusion; If you park hop straight from park to park, the transportation is quicker and 100% worth it during the peak season. Easter break is busy and nobody wants to stand through security check and ticket lines twice in a day.
If you park hop, but go to the resort in between, like we did, obviously it is not worth it. If you feel your plans could change as ours did, again, probably not worth the purchase.
However, If you are going to Disney and the park is not crowded, I would have to say this is definitely not worth it. Friends of ours didn't purchase it, as they were there just for a couple days visiting us. The 2nd time we park hopped, we went from Epcot, to Hollywood. Our friends took the Boardwalk/Swan & Dolphin boat from Epcot, to Hollywood to meet us at guest relations. We Literally waited for them for 5 minutes, and this was during busy peak season, plus, they had 3 stops on the way. When we got to the transportation, we had to wait 20 minutes for it. So if it only saved us 5 minutes during peak season, I think during the less crowded months, purchasing this would be a great waste of money.