The streetcar right of way has totally helped revamp St Clair. It's fantastic.
Like the others who said it, the consistency/reliability is the best part. On weekends you feel like you're flying down to Keele it's so fast.
Capacity is sometimes an issue - it gets really busy going east at morning rushhour around Christie, Wychwood, Vaughan, Bathurst and if you're trying to go west during afternoon rushhour each streetcar pretty much fills up with subway passengers at the St Clair subway stop (Yonge)
Driving on St Clair isn't terrible and nowhere near as bad as some people sepculate. The biggest problem is that there are too many stop lights between Bathurst and Landsdowne - Vaughan, Wychwood, Christie, Arlington, Winona, Alberta, Oakwood, Glenholme, Northcliffe, Dufferin, another after that, and then Earlscourt I think. That's 12 stoplights for less than 3 kilometres.
But if I have to go west by car I'd take Davenport, St Clair, Dupont in that order of preference. I'd prefer it to Eglinton anyday.