The EZ Pass transponders have an antenna that needs to be HORIZONTAL to get picked up. Just chucking it on your dash willy nilly may result in spotty results.
Maine has no maintenance fees, charges in $20 increments, keeps a $20+ ish balance, and the transponders are $10. I save on the ME Turnpike, but have also saved, IIRC, four cents in NY. NY credits the money saved on toll collectors, apparently, but doesn't give me the resident discount.
The transactions aren't "live", they get reconciled approximately daily, so they want drivers to have a cushion of money to cover things. It's also probably easier on the credit card fees to charge in slightly larger increments.