Everybody's pretty much summed it up, but here's how it works for us:
We don't do the dining plans. I have done dining plans, and I've kept close track of expenditures on trips without dining plans. I've found, for the way we like to eat, that I always save $200-$300 per trip when we DON"T have a dining plan. I also don't purchase my park admission through Disney; I get better deals through Tickets at Work and
Undercover Tourist.
We hate the Disney buses. I abhor having to wait 30-40-60 minutes for a bus at park closing, when EVERYONE else and his brother is also in line for a bus. Then they jam you in like sardines and you get to stand on the bus, too. Totally ruins my Disney experience, so even when we stay onsite, we rent a car- so that makes the cost of a rental car inconsequential for us. I can usually get a pretty good deal on a rental car by stacking membership codes and coupons, and checking prices regularly. Regardless of where we stay (on- or off-site), I like being able to make a grocery store run, having my own vehicle parked outside my hotel room, and not having to stand in line for a Disney bus. I DON"T like having to pay $20 a day to park, though!
We enjoy late night EMH, and that's one of the true losses for us when we stay offsite. We deal, though. I also haven't found it a big hassle to have to wait to 30 days prior to the trip to reserve FP+, but we've been to disney 25+ trips and know that we'll always be back, so there are no "do-or-die, only trip ever" must-do attractions for us.
We love, Love, LOVE the extra space, extra bathrooms, (sometimes) private pool,
and money-savings of renting off-site, especially a condo or home (and it's even cheaper if we travel with friends).
Having said all this, our last 3 trips were onsite, twice with a rental car and once without. It all comes down to the budget, the number of nights we are staying, the number of travelers, and what kind of deals on a Disney room I can find (like at Orbitz... they have the Disney discounted rooms if there's availability, PLUS they will let you use their current discount codes- and if I go into Orbitz through ebates, I get whatever cash back deal Ebates is running at that time.
SO... that's how it shakes out for us!