While ICBC is always responsible for mandatory coverage (essentially third party) you can chose another insurer for the rest of a the comprehensive package (excess liability, collision and so on). I have used Canadian Direct for the optional part since 2002 and they have always been very competitive. However, I do not take that for granted and when I get the renewal notice from CDI I ask the broker - you have to use one to get the required ICBC renewal anyway - what a private insurer and ICBC would charge for the same coverage. To ensure that I am comparing like with like, I take a printed copy of the declaration with me.
For the last ten years CDI coverage was cheaper than the competition. That is no longer the case. Not only does another private insurer offer a lower premium, so does ICBC! This seemed so unlikely, based on ten years experience, that I called CDI to see if there was a mistake, or if they wanted to make me a better offer. The answer was no and no.
I am not going to reveal my claim record or the quotes I got since your mileage will vary, but I do think that is worth doing your due diligence. You will probably save yourself some money if you get more than one quote for your optional insurance, and it need not take you out of your way or cost you anything to find out.
By the way, when I changed my address from a townhouse in Richmond to a condo in Vancouver, my private insurance premium increased. I thought this a bit odd since at the townhouse it was simply in an open lot, with a roof. Like a common car port for residents, it offered little protection. At the condo it is an underground garage - with locked doors, and a car entrance requiring a programmed remote. So I would have thought better protected against theft or vandalism. Well, I was wrong about that too. One of my neighbours - who has a Mercedes - found the driver's window of her car smashed and the contents rifled. And apparently that is not the first time such a thing has happened here.