We are transiting Ottawa-YUL in April. and are taking VIA Rail.
The cost $35 one way) is well worth it, and there is frequent (20 minute) bus service b/n Dorval VIA Rail Station and Trudeau Airport (YUL).

FYI, a ? arose in another thread about the bus which leaves Ottawa train stn. daily to connect with the AF flight to CDG. I found the answer on Trip Advisor:

http://www.tripadvisor.ca/ShowTopic-...a_Ontario.html

In a nutshell, the bus is for AF passengers only, and is treated like an AF flight!

ps The TA thread also discusses the fact that airfares to Europe are cheaper ex. YUL than YOW - which is why we are taking VIA Rail!