I've taken a "luxury" bus from the Boston area to NY once. I'll stick to Amtrak although, given where I live, it's not very efficient. Boston area to DC really takes too long though I've done it.

I hate driving into NYC though I've done it with someone else (or because I was headed somewhere else afterwards). As you say, with multiple people, the numbers don't really pencil out--especially given it takes longer for me.

I do BOS-NYC occasionally. Have driven, taken the train, and have taken a bus. The train was fine, but the bus was also fine, was far cheaper and the bus terminal is just as central in Manhattan.