I'm currently finishing my undergrad at Brandeis, and I've lived in Waltham (on campus) for most of my time here. I wouldn't say that Waltham is impossible without a car - it really depends on where you live. Although grad students don't live on campus, I know many grad students who live within a 10-20 minute walking distance, and they seem to get along ok without cars. Brandeis has shuttles that run during the day/night, and while I've had some mixed experiences and can't say they're extremely reliable, they can be helpful. The commuter rail is really convenient - it's always on time and has a fairly decent schedule (runs pretty frequently). The only issue with it is that it's expensive. It's about 6 dollars for a one-way trip from Brandeis into North Station (Boston).
In terms of commuting to/from Brandeis and Porter Square/Somerville, the commuter rail goes directly to Porter Square, so that shouldn't be an issue. Once you're in Porter Square, you can take the red line and fairly easily get to most places in Cambridge. I would check out the commuter rail schedule on the MBTA website (Fitchburg line) so that you can get a sense of what the schedule is like and whether you think it would work for you.
Let me know if I can provide any more help, and feel free to message me!
There is some stuff going on-- decent restaurants, grocery shops, a second run movie theater. But it's mostly a quiet suburban town. It would be really hard to live in Waltham without a car. There is not any public transportation around Waltham, and everything is pretty spread out. If you live in Boston (or more accurately, Somerville or Cambridge) you definitely don't need a car. If you're looking into living in the city and commuting (which about 60% of grad students do) check out these neighborhoods: Inman square, union square, porter square, teele square, Davis square. PM me if you have any questions!
