We have gone on two trips this year and stayed both off site and on site. I enjoyed both, but when we go back next Jan. we'll be off site again. It comes down to a few deciding factors for us:
1. Stay offsite means we can have a longer trip. In Jan. we will be staying 9 nights in a 3 bedroom condo at Windsor Hills. My parents will be joining us for 5 nights, so the whole family will be able to stay together for one price. My parents wouldn't even have to pay, but since they want to, they get a very reasonable daily rate from me.

If we stayed onsite, I'd want to be in a mod or a deluxe, so it would cost considerably more, even if we got a good discount.
2. A condo gives everyone more space. Especially when we're there for longer trips, I like to be able to put the kids to bed in one room and still be able to enjoy some reading or TV in the living room. When we stayed on site, it was a constant balancing act of getting the kids to sleep at a decent hour without Mommy laying awake in the dark for hours.
3. I like having a car. My dh is not in agreement on this point.

However, I hated taking the buses everywhere. I don't like waiting for them to show up either. I want to jump in my car and go. When we stayed at Windsor Palms, it usually took about 20 mins to get from the front door to the turnstiles at the parks. Most of our bus rides were just as long (and some were significantly longer when we stopped for more pick-ups). It took us an hour to get from POFQ to the Poly for brunch one day since we had to transfer to the monorail at MK. I was expecting the travel time, but it didn't make me like it any better.

For our next trip, I have promised that I will do all the driving since my dh did like Disney transportation.
4. I love having a full kitchen. When I'm relaxing after a long day at the parks, I don't want to have to run to the ice machine or the food court to get what I need. We kept some snacks and drinks in our room when we stayed on site, but it just wasn't as convenient. I also like being able to come "home" and pop a frozen pizza in the oven if we don't want to eat out.
I know I'll stay onsite sometimes because, it is fun to be at Disney 24/7. If my dh and I ever go on an adults only trip, I'd love to stay at the Wilderness Lodge or the Polynesian. But for those longer trips, its off site for me.