We picked ours up at a New Jersey rest stop a few years ago. Another poster said theirs didn't work right away, ours worked fine. I remember something about 48 hours, I think it was that by the end of 48 hours you were supposed to go online and "attach" a credit card number to the card, and record the vehicles that would be using the EZPass. A $25 minimum is maintained on the EZPass. Monthly statements are emailed to you if the pass was used in the previous month. Once you figure it out, it makes sense how they do it.
So far, our EZPass that we bought in New Jersey works in any state that uses EZPass (if that was a concern).
I don't remember how many vehicles can be listed with one pass, but it is a lot. Just put the pass in whichever vehicle you will be using - if we don't have the velcro on the windshield to hold it, we just hold the pass up against the windshield when going through tolls. So far it has worked fine.
I need to go check, but I think I registered my F250 as pulling a TT on the EZPass. There was something about trailers / campers when I was setting up my account.
Biggest "problem" with EZPass? You don't realize how much you are paying in tolls until a month later when you get a statement.