We've done the drive twice- once in 2005 for our flight to MCO to go to Disney, and once in March of last year to FLL for a
Royal Caribbean cruise (I know, I know...I REALLY want to try a DC some day!).
We did the Woodstock route in 2005, and it was a fine drive- good weather all the way. Our car had some troubles, but we took care of that at the Toyota dealership in Woodstock. Truth be told, I found the scenery a little dull, but almost anywhere in NB/ME/NH you drive that isn't near the water is like that! All woods!
In 2006 we went across through St. Stephen and Bangor as we usually do (by the way, we live in Saint John). Are you flying Southwest out of MHT? The best advice I can offer is print off your boarding passes at home or at the hotel business centre 24 hours before your flight. You'll (hopefully) get an "A" boarding pass and get to board first (there are usually lots of people with kids going to MCO, so they'll get to board first).
As for the OP who asked about flights being cheaper...YES! Even though it's a ~6 hour drive from Saint John to Manchester, if you add gas (maybe $100- we have a small SUV), a night in a hotel at each end of the trip (say $200, including free park and fly), and whatever wear and tear is (I have no idea!), you still come out ahead. If you got a deal like we did, 4 adult tickets from MHT-MCO came to $800 taxes, fees, etc., included. I was looking at the Westjet site, and the price PER PERSON for the same time period, Moncton to MCO, is $1200 ($600 each way). AC charges $1150 round-trip per person from Saint John to Orlando at the same time. So, you definitely come out ahead by driving to Manchester. Of course, it is a long drive, so it might add a bit of time onto your trip if you live farther away than SJ, have small kids, like to take frequent rest stops, don't like driving at night, etc.
Hope this helped! Feel free to ask any questions.
Starrzone