We've done it...hmm, 3 or 4 times now? Twice in Jan/Feb, and of those drives, three were clear dry roads and skies, and one was white-knuckle awful. We are doing it again in Feb, though, just planning to leave early and/or take our time back if the weather is bad.
Clearing customs at the border in our van is *always* easier than in Toronto airport. At least if you wait (and we don't usually), you're in your car, the kids have a movie or a snack, no-one has reason to whine and there's no luggage to stress about. Honestly, I can't say we've spent more than maybe 30-45 minutes waiting, and it's usually more in the range of 10-20 mins. (Truly less hassle and time than collecting the luggage, going through customs, and dropping the luggage off before getting our connecting flight in Toronto.) That said, we aren't usually in the line on Friday evening before a long weekend, either.
Flying domestic has been easy peasy too. We have our passports, of course, and they are so easy to use as airport id.
THe other part of it is, my dh LOVES road trips.

He may be tired at the end of the driving day, but he enjoys it. Used to be, we'd fly from home and then drive as a vacation (like the time we drove from Tampa to Louisiana and back). With kids in tow, I am more interested in "stay put" vacations, so he gets to drive his heart out on the way to and from the airport, and I get to settle into a resort once we get there. Win-win.
The fares don't even begin to compare. I'm paying more than I like in Feb, and it's under $1100 for four. Add the drive and the park and fly and the extra food for road-days and it's still well under $2k. The *cheapest* fare from YHZ I could find for close to our dates was more like $3k. And dh still would've been looking for somewhere to drive.
We love to fly from Manchester NH. We get great fares and service on Southwest, and we don't have to drive in to Boston. (Continental and a few other big airlines fly from MHT. We have flown Continental but prefer SW.) It's a great little airport. From Halifax to Manchester is about 10 hrs but we never make it in that. We'll leave home one day and overnight with family in NB, then get to Manchester the next day. Coming home we'll usually just do one overnight. With kids, we like to have proper meal stops, and stretch our legs.
I think it depends on your family. For us, it's more than just the money. We enjoy the little driving "adventure" at the start of the trip. But the money makes it an easy decision for us. YMMV.
Good luck making your decision.