Yes, but it sucks, I hate doing it. If you're going to drive to YYZ and pay for parking it's almost as easy to go to DTW or BUF. Unfortunately YXU is somewhat equidistant to all three major airports.

Fares out of YXU are generally high - AC makes good money on this route. You can see this if you compare from YXU vs. YYZ on many fares; sometimes the price doubles. You are really paying for the convenience. I use flight passes often so it costs the same regardless of whether I fly YXU or YYZ. Even without a pass, by the time you factor in parking, rental car and gas, you're not paying much of a premium.

Your most convenient option is YXU-YYZ-YXU, but it's not cheap. YYZ-YOW-YYZ is usually not terrible if you want to avoid customs. BUF has the best MR deals... you can easily score 2000 EQMs in 4 segments for under $200. Sometimes you can find okay deals with Expedia.ca flying YXU-DTW with Northwest and then doing a MR on *A carriers, but it depends on how much that roundtrip to DTW is.

I would agree with others though on the benefit of getting Prestige... it's nice if you get it, but I'm not sure why you'd MR for it unless you know you'll use the upgrade certificates on Latitude fares. I was an AC Maple Leaf Club member this year (and was planning on joining through CIBC since it is a lot cheaper next year), but I ended up with Elite instead. That was $400 well spent... mostly relaxing in the YYZ MLL waiting to catch my YXU connectors