My experience is similar to lostinthewash's.

I haven't been selected for the scanners for US bound flights so I can't comment on the opt out experience there.

Domestically the opt-out pat down is not quite the same as the one I experienced in SFO. It's not as intrusive (no waist band or collar checks that I remember).

By far the biggest difference is the interaction with the CATSA people. Almost 100% of the time I am referred to as sir. I've never heard one yelling yet (100+ domestic flights this year). Are they perfect? No. I got mildly annoyed at one XRay operator who insisted that I had a lighter in my laptop bag. She was willing to let me walk away, but I was tired and quite irritable that night so I challenged her on it. Her co-workers unloaded my bag and when they found nothing they sent me on my way and had a discussion with her about what they found and how to better spot things the next time.

The "lucky mat" (or dance-dance-revolution mat) seems to be random. I've had time in line on a couple occasions to see if I could notice a pattern. After 40-50 people there seemed to be none, but about 1 in 6 was getting sent to the scanner. As lostinthewash said, everyone is told of their option to opt out. If you choose to opt out, my experience is that there is no yelling, no incredulous look from the screener and no sense of animosity. In fact when I opted out the second time (same airport) the pat down person said "Hey I know you from before" and proceeded to indulge me in a fairly lengthy, polite and informed conversation about the process.

Overall I'd give my Canadian opt-out experiences a solid 4 out of 5.

---jeez the mods are working fast in here today....already merged and making me look like I want to talk about it twice as much as I really would