I just took the Maple Leaf both ways from Toronto to Rhinecliff, New York, and back again. The postings here were very helpful. Here are my observations:
Book Business Class. It's only $25 more each way. You get more seat space and you're right next to the cafe car, such as it is. Also free Perrier! There are electrical outlets for every seat in Business Class if you want to plug in a laptop or whatever.
Bring your own food. It's a cafe car, not a dining car. The food is all pre-packaged. The cheese and crackers tray is OK, but the burgers and sandwiches are just popped into the microwave. There are chips, candy bars, juice and booze available as well.
Border crossing at Niagara Falls (both Ontario and New York State): expect a 2 hour delay each way. The train crew changes and the cafe car is restocked. You cannot get off the train and you can't use any electronic devices while the customs inspection is taking place. On the way back, we went through it twice: once on the American side and again on the Canadian side.
Air conditioning: it's too cool for me. I brought a Pashmina shawl and that was perfect for wrapping up in.
Scenery: it's not really interesting until after Buffalo. When the train turns south at Albany, then you are following the Hudson River and that is quite scenic.
It is a long trip but I found it quite relaxing. I only went as far as Rhinecliff NY, so I can't speak about the trip all the way to New York.