Air Canada has operated their CRJ-100s as mainline since 1995. It was their newhire aircraft, supplanting the DC-9 when it entered service. However, it was a difficult transition for some, as the captains were all widebody FOs with relatively little experience in a "hot" aircraft such as the CRJ, and of course the newhires were coming out of Metros, Dashs, Twotters, etc etc etc. Nonetheless, the CRJ was instrumental in AC becoming the dominant carrier between the USA and Canada, employing the RJ years before any American competitor.
2-3 years ago, Air Canada Regional (Jazz) ordered some of Midways' former CRJ-200s and has since shared the CRJ duty with AC mainline, thus taking some expensive AC mainline crews off the RJ. Eventually, AC will relinquish all CRJ, including newly ordered CRJ705, really the CRJ900 with a 1st class cabin (designed to replace the BAe 146s currently with Jazz), to Jazz. AC has also ordered EMB 195s, but it has not been decided who will take these. Smart money says AC mainline, as the EMBs are almost identical in capacity to the DC-9-32s AC used to operate until 2002.