I bought mine online and just took the voucher to guest relations at the front of MK to get the actual pass. The pass isn't activated until you exchange the voucher for it. I bought the pass in April, but didn't pick it up/use it as a ticket to get in until Aug. 8, so it expires this Aug. 8.
If you know you're going to be making more than one trip in a year I think it's worth it. I'll have used it on three trips by the time it expires, so I know I've saved some cash over the regular ticket price.