Open Left lists the high-speed rail projects most likely to be funded with stimulus spending - and while Boston to Montreal (and Portland) were on the list, it wasn't very high up (not that we should be greedy: Boston-NYC-DC is #1 on the list).
That said, I still found the whole concept of the project interesting. Would I drive or fly to Montreal? Probably not. But I'd consider going there for a long weekend trip if there were a competitively-priced train. I bet a lot of Montreal residents would think of coming to Boston too.
I decided to check out the website for the plan, just finishing the detailed summary (pdf file) of the Phase 1 study. It predicted over 680,000 users annually starting out, more than 220,000 of which would go the full route. It's important to note I only read about the Boston-Montreal line, but the overarching plan would also link Boston to Portland. Furthermore, given the commute potential, a little publicity and future smart growth, it's easy to foresee a scenenario where many more than 680,000 people use the system. Linking Boston to places such as Lowell, Nashua, Manchester and Concord has a lot of potential.
I would love to see this take place, more to extend commuter options and public transit within Metro Boston than any other reason. I'm sure the people of NH and Vermont would feel similarly about their localities. Plus, it'd be nice to go visit Montreal using the train - my friends have raved about the city, so I'd probably go there if there were better travel options available.
Update: A few quick facts. The train would travel at an average speed of 110 mph. That's relatively slow compared to some other planned supertrains, but according to the report, it's the most feasible option. The safety requirements for trains that travel in excess of 150 mph are such that it would require significantly more investment to make this plan feasible.
Furthermore, to get passed the 150 mark, the added cost to the project would mean far more expensive fares for users - which was actually projected to reduce ridership. At 110mph, the costs for users would be around 20 cents/mile, which amounts to $66 each way. That seems low, but even if it's a hundred or a little more, that's much better than spending the $600+ for roundtrip airfare. Heck, it would even give Greyhound, charging $180 roundtrip ($92 if it's booked early) a run for its money, while saving users 2-3 hours each way on the 8 hour bus trip.