There's lots of information about this topic to be found online. This is one of the better sources I came across:
http://www.bdo.ca/BDO/media/Tax-Factor-Banners/US-Tax-Issues-for-Canadians.pdf
My rough non-professional understanding:
As a Canadian resident who owns US property, your US rental income is taxable both in the US and in Canada. Moreover, in the US, an agent who rents your property on your behalf (eg. DVC store) is required to withhold taxes on that rental, at a rate of 30% of the gross rental income. If they don't withhold, and you don't file, they can be held responsible for those taxes. Therefore, some DVC rental agents have begun withholding, even on small transactions. The withholding tax is generally more than your actual taxes owing. If you want to recover the difference, or reduce the tax withheld upfront, you'll need to file a US tax return. In either case, your rental income must also be reported on your Canadian tax return, and some or all of the tax you paid in the US can be recovered as a foreign tax credit. It's not entirely clear to me why you should care about the 30% withholding rate if you can claim it back on your Canadian tax return, but I think the catch here is that you can't claim back more than the Canadian tax owing on that particular income, which may well be less than the 30% withheld.
Somewhat off-topic, but much the same principles apply on the sale of a US property.
Personally, I think the whole thing is just too complicated to make renting worthwhile, unless you're prepared to do it under the table, with all of the legal and ethical implications that implies.
I meant more for IRS - do you file as a Canadian for rental income? Heard once you file, you have to file every year forever.
I don't think this is true. Once you make an election to file rather than having taxes withheld at 30%, that election is apparently "permanent" (or not easily changed, anyway), and you are then required to file as long as you have income to report. But if you don't rent your property in a given year, or no longer own US property and have no other source of US income, then there would be no reason to file.
https://www.irs.gov/individuals/international-taxpayers/taxation-of-nonresident-aliens
That said, I'd have to be losing a pretty good chunk of money before I would think it worthwhile to apply for an ITIN. That just feels like a can of worms that I'd rather not even open.