We've used the Diamond passes for most of our trips. If you have a large stroller, a child asleep in a stroller or a lot of stuff in your stroller - it can be a huge luxury to not have to fold the stroller to board the tram to get to your car. You can usually park significantly closer (next to handicap) at HS, Epcot and AK. I am not sure about MK - we always stay on site and don't drive there, but do drive to all others. We pretty much always arrive at rope drop, go back to the resort for a nap, then return for dinner/evening. We usually travel in the off-season (April, May, Oct, early-Dec), and have never found the Diamond parking to be full.
The passes are good for a year - if you have any friends who have had them, you might see if you could borrow one. I have also seen them for sale on e-bay. (FYI - While I'm not sure that this is legit, there is nothing personalized about them that would make them not be able to be sold. We've loaned ours to friends before.)
Hope this helps!