We really should make this topic a sticky. Seems like I've answered this 100 times.
The following info assumes you are a carrier. If you are leased or a company man, the carrier may already have done some of this stuff for you.
The good news is, it takes us only about 60 seconds to clear customs coming into Ontario or Quebec. Once in a while they'll send us to x-ray but not often.
It takes about 10 minutes to clear going into the US and we get x-rayed every time.
The bad news is there is a bunch of hurdles you need to clear or it will be a nightmare. I suspect the stories you hear are from people who didn't clear these hurdles.
Other bad news is that southbound rates suck.
Here we go.
If you've been in trouble with they law you may not get in. Even a DWI 10 years ago and they will turn you around.
No guns.
You should get a Canada Customs Carrier Code. If you cross more than 5 times a year you must have it, otherwise you will have to park the truck and go inside to see the shippers customs broker. This is a timie consuming hassle. I recommend getting the CCCC. Once you get the CCCC you will need to spend ~50 on getting the bar code stickers printed.
Commercial Carriers
To reenter the US you will need to contact the NMFTA and get a SCAC code.
Also to reenter the US you will also need to apply for an ACE account and complete an online ACE manifest each time you enter the US. Obviously, you will need a reliable internet connection and printer for this.
For ON you will need a C.V.O.R.
Commercial Vehicle Operator's Registration (CVOR)
For QC you will need to have your trucking business registered with the province. Costs ~$75 IIRC and you will need a QC mailing address because they will not mail outside of QC. If you don't have a QC mailing address , you will need to designate a power of attorney. I think Nova Permits charges ~$350 to set you up for QC, including $100 for the power of attorney service. If you decide to go ahead with this, let me know and I'll do it for $200.
Allowable lengths, axle weights etc etc are regulated by each province and they are all different.
ON and QC now have a speed limiter law requiring any truck mfg after 1994 to have the max speed set to 105 kph.
Hours of service rules are federal and are slightly different than the US.
I can't speak for the other provinces.
Good luck.