There are several AC Maple Leaf Lounges at both YUL and YYZ which also permit STARGold members to make use of them. There is no charge for alcohol, and each has an open bar with local microbrew drafts on tap. [Not a very good snack selection beyond the nuts, pretzels, cookies and cheese|crackers.]

At both terminals/airports, you would be using the Transborder MLLs, which will be located after you have cleared US Customs|Immigration. AC also has Transborder MLLs at YVR and YYC servicing their own and UA flights into the US.

Not sure how close to gates the lounge is, but YUL does not have a large transborder wing, so it can't be that long a walk. YYZ's transborder wing tends to sprawl, so walks to gates can be a fair trek, as can be those at YVR, particularly if you are on a commuter AC Regional|AirBC -- now called JAZZ [don't ask] -- flight to SEA or PDX.

I would say the two lounges I have used [YYZ and YVR] are as good as any RCC, and most STAR partner business class lounges. Several PC terminals are available for free internet use, along with workstations where you can set up your own PC. [Operated as Xerox Business Centres.] YYC is a bit cramped and dated -- it used to be the DL Crown Room -- but I suspect the one at YUL should be as nice as the domestic|international one there, since this is AC's headquarters city.