Hi...I just did what you did for the second time last month (my first time was in '08).

Time to get thru customs is TOTALLY dependent on how many int'l flights happen to be arriving at roughly the same time as your flight. My first time in '08, I happened to hit it as the only flight at the time; I was thru in less than 5 minutes. This time, there were multiple flights and the queue was about 200 people deep (which is not bad...there is room for MANY HUNDREDS more in the customs hall); took about 40 minutes. It's a LONG walk from the arrival gates to the customs hall...if you are able to walk briskly, when you reach a point in the hallway where you can do so, walk PAST the slower walkers...the more people you can bypass, the shorter your wait will be to see a customs officer.

The officer will ask you how long you intend to be staying in Canada and the objective of your trip, where you will be staying, and possibly what you will be bringing in with you, etc. Depending on your answers to the initial questions there may be more follow-up questions, but it's quite straightforward...be honest, friendly, and businesslike and you'll have no problems.

You will pass thru customs before you retrieve your checked baggage. From the customs hall you will then go down one level to baggage claim. Int'l baggage delivery CAN be slow at YUL; last month even after the 40 minute queue my bags were not out yet when I got to the claim.

Upon exiting the baggage area, you will hand in your Canadian entry form to another officer stationed at the exit, and then you're on your way. If you are considering the 747 Express Bus to downtown Montreal, this is where you will get your ticket...at the currency exchange counter just after you exit the baggage area. There are one or two good threads here on this forum with details on this new bus service.

For your return to the States, yes, arrive early. You will fill out an I-94 form for re-entry which you must present along with your passport to a US customs officer. Again depending on timing, you might get through within minutes, or it might take MUCH longer...allow for the longer extreme and just bring plenty of reading material with you in case you get through fast and end up waiting around for 2 hours. For re-entry back to the States (which occurs at YUL...when you disembark from your flight at ATL you are domestic at that point), you may be interviewed briefly by an officer while in the queue, and then the more detailed screening at the officer station. US customs officers are necessarily paranoid what with conditions these days, so be prepared to be viewed suspiciously even if (though) you likely have nothing to hide...again, just answer truthfully, in a friendly but businesslike way, and you'll be fine. (I traveled and toured Montreal as a solo traveler and the customs officers were all suspicious that I was traveling alone and didn't "know" anyone; I just stuck to my honest story and eventually got through after a few extra questions.) Once you've passed through US customs, you'll be in the int'l departure area; there's a Duty Free store immediately past customs, and then past that the usual complement of newsstands, food vendors, etc...either of these options is a good place to dump the remainder of your Canadian currency. Spend ALL of your change if you can (even if you have to count out all you've accumulated...darn those loonies ($1 coins) and twonies ($2 coins) anyway); most currency exchanges on either side of the border will only exchange paper currency.

It's not clear from your post if your purpose in Montreal is business or pleasure, but in any case have a GREAT time...Montreal is a true treasure. Tour well, and eat well...get off the beaten path and explore more of the city than just the typical tourist sites - you will be well rewarded.