EZ Pass is great! Of course we're on the MassPike, so use it pretty regularly. But given the time savings (and the toll savings at the Allston tolls for us), and the fact that the transponders are now free, I have been wondering why so many folks with Mass plates are backed up at the cash tolls. We use ours to go to Grandma's in Maine pretty regularly, and now in NH you can go thru at 60 mph (love the no slowing down business).
Bus for a drive down the east coast like you're doing, I would definately recommend it. You might want to think of getting it from a toll-road authority you might be using regularly that might give a discount for transponder use. We have both the FastLane pass for Mass, but also an EZ Pass for Maine which gives a discount at the York tolls on the Maine Turnpike (what we refer to as the "Getting out of Maine Tax"). I think some of the states along the way offer a discount for using the local EZ pass, which might be worth getting your EZ pass from them instead of Mass.
I assume the OP will be hopping on 95 thru RI and CT, but if you can get off of 95 in that stretch of CT heading in to NY, it'll make driving more pleasant. We like the Merrit Parkway for pleasant and pretty driving, while the parallel stretch of 95 is ugly and full of trucks. Also during any sort of busy time (any time but late at night, basically), we have usually found that we save time looping up to the Tappan Zee bridge rather than the George Washington.
Have fun!