The window version works fine. I got a suction cup mount and can remove it when I leave the area. I install when travel back through the area and remove after leaving. The Suction Cup Mount also allows me to remove and keep in my bag if flying. I can set the EZ Pass up for a rental car and remove when the car turns in. Here in Texas they use a NON removable transponder that has to be removed and deactivated with each vehicle.
I can personally speak for the fact that the EZ Pass works for the entire system at least the Northeast Portion including New England, Mid Atlantic, Ohio, Indiana, Illinois, Wisconsin if it stretches that far yet. When I set up my account Ohio was in the system so I was able to drive from VA-MN and not miss a beat. Some of the more modern designs allow you to maintain normal interstate speeds by use of dedicated EZ PASS travel lanes. I got a pack of two from Amazon for around $10 bucks just a simple molded semi-flexible wire and a suction cup.
If it is a rental I put it on the drivers side of the inside mirror as to reduce risk of forgetting it. For my own vehicle I put it on the passengers side since the NTTA Toll Tag is on the drivers side.