Currently there is a 30% discount for NH based E-ZPasses on NH tolls. There is talk about a commuter discount program that would cap the total # of tolls you'd have to pay in a month. Time will tell if that gets implemented. The bill this session didn't go anywhere.
You'd want an E-ZPass if you were going up I-93 to Concord. They're currently working on installing open road tolling at the Hooksett plaza, so by next summer those with an E-ZPass will be able to go through at highway speed. There is currently ORT on the I-95 tolls in NH as well. This severely reduces the delay to get to the delay at the Maine tolls.
There may at some point in the future be an anonymous E-ZPass transponder you'd be able to buy for cash and reload with cash. The only downside to such anonymity is if your transponder does not get read (they do claim 99.99%, but I've had my E-ZPass not read a few times, and I don't use it that often), then you'd get a toll violation. Currently the violation is the price of the toll plus $1 administration fee, so long as you pay it in 30 days. If you have a normal E-ZPass, you register your car's plates to it, so if the transponder does not get read, they match your plate to your account and debit that rather than giving you a violation.