I find the MAJORITY of my letters and post cards TO Canada and the US simply vanish. But stangely enough, almost everything arives here with no problem, including various parcels that have been sent for Christmas/birthdays/etc.
i think it is a it of a crap-shoot, unless you fork out for FedEx or UPS.
In over a year, we have received many packages from the US. Only one was lost, the one without a tracking number. We have sent mail and only some arrived. If you send something priority mail with a tracking number, even if the address is incorrect or incomplete (this happened to us), with the tracking number, you should still be able to track it.
Just wanted to say that I still love having my Earth Class Mail service. They scan your US postal mail and put it up on a web site for you to access. Then you can batch it up and send it in shipments... has worked beautifully for the last 2 years though their prices have increased significantly. http://www.earthclassmail.com