I only use my EZ Pass a few trips each year but it is well worth it in my opinion.
Each state EZ Pass is a little different. Example, the Pell Bridge in RI is a $4.00 toll with or without EZ Pass, but RI residents pay 83 cents with EZ Pass. The RI system does not charge for inactivity but some states do charge for inactivity. If you have a New Hampshire EZ Pass you get a discount on tolls even if you are not a New Hampshire resident, at least they used to, I don't know if that has changed recently.
I only have one transponder and all the places I have gone so far will automaticaly figure out if I am towing and charge acordingly. Some states do require a seperate transponder if you are towing.
It would be best to check the rules for the states you plan to travel the most and see which state EZ pass (or Illinois I-Pass) would be best for you.