I have done both, and generally we prefer onsite - UNLESS - unless you are going to do stuff in the other Orlando, or you want an inexpensive room for your arrival and departure days, or you have a spouse that is in Orlando for work and you can freeload in the same room
A free room is always a no-brainer.
What I found was, this:
1. Most of the offsite eateries are chains. I'm not so into Perkins, Golden Corral, Denny's, and the like. If those are right up your alley, then offsite might be a good choice for you. I suppose we saved some money getting groceries, but they weren't free and we were locked into eating them once we bought them. Later it the week it was, "Okay, somebody's got to drink this orange juice [yogurt, cereal, cookies], or we'll have to throw it out!" On property, I pick what I feel like eating each day.
It's still worth getting a box of cereal or case of bottled water even if you don't eat all of it, but we did end up tossing some food...Also, there are plenty of eateries in Orlando that are EXPENSIVE. Sometimes it can be fun to eat offsite, but we dn't usually save much money when we do.
2. Transportation. The last time we stayed offsite we were at the Vistanas, which was very close to the entrance of WDW, but maybe not the best property entrance. It still took us a while to get to the parks. The 535 entrance has much better park access - at least for getting to MGM and Epcot. Driving can be confusing at WDW. Roads are marked, but not always heavily - miss one sign and you'll have to make a large loop around the property to get where you want. Off property, someone's always got to be the driver. They've got to be sober, they've got to be awake, they've got to navigate, they got to avoid speeding tickets. Disney busing isn't perfect, by any stretch, but I usually prefer it - and I'm no bus fan in the real world. Everyone in the family gets to relax on Disney transportation.
3. Conformity and quality of customer service. The Vistana was certainly a nice place. We had lots of room, it was very clean. The pool was nice, it was secure. It wasn't as cute as Disney. There was on Mickey soap, no Mickey wake up call, nobody said, "Have a magicl day!"
Off property, you just don't - generally - get the same customer service. That's not to say that the folks at the Vistana weren't nice, they were, but at several of the places we dined we got less than stellar service.
At WDW, when there is problem, the staff is usually willing to help solve the situation quickly. Once we dropped something while getting on the monorail. It went UNDER the monorail car. On a fluke, I mentioned it to a CM at the station where I got off. She took my name and lo and behold - the item was waiting at Guest services a few days later when we dropped by!!!! I'm sorry, but there's just no way that would happen at a subway or train depot anywhere in the real world!! I feel lucky if I can get a subway employee to tell me how much fare to buy!
4. Magic motivation. But the real difference btween on and offsite is hard to describe. When we were on property, we spent a lot more time doing things. Our room was nice enough at Vistana that it was hard to get everyone up and out - and we kind of felt like we all had to go together. We had a group of nine, and at the VERY least - we had two cars so we HAD to come and go in groups of four or five.
On property, we still mostly traveled together, but we didn't HAVE to travel together. When my mom got tired, she could go back by herself if she wanted. If my sister wanted to sleep in, she could.
At the Vistana, and this is where it's hard to decribe, the kids got sucked into watching tv in the room a fair amount. I don't go on vacation to watch tv. I can do that at home. When we were on property, they weren't interested in watching tv. In fact, we hardly turned it on at all in our room.
Now if you are the kind of folks who like sitting in your room watching tv, eating in your kitchenette, playing tennis or something like that; then I can see how that would appeal to some folks. Me, I want to be on the go when I'm at WDW. And I want to be on the go in the parks all day and night.
However, the year before, we waited for an hour plus to get on a bus to our hotel, mulitple times. (We were there in Sept/Oct) We ended up giving up on WDW transport and driving the last half of our trip, I find it 100 times easier to drive, even to the TTC. At the end of the night, sometimes the tram lines get a little long.....but atleast there is the possibility of walking to your car, not so with the busses. You stand, and you wait. We only had an issue with this once, on an MK night. We just lingered and walked slowly out of the park. Then we just walked over to the Contemporary, got on the monorail and back to the TTC. Tram lines were almost done, but we were close, so we just walked to the car. All other days, we had no problems at all. 
Obviously, there are many factors. I can tell you, OP, that if you stay at Windsor Hills, you will probably have an AWESOME time, just like we did. But, I can't tell you that ANY off-site vacation you have will be the same way, b/c it all depends on where you stay, IMO. Quoted Poster obviously didn't stay in the best of places convenience wise, and probably was against it from the start b/c of those Disney themed rose colored glasses diehard on-siter's tend to wear. (I used to own a pair of these, btw. 
The magic is not just Disney, the magic should be spending quality time with your family. Disney is just a setting for this, some of our most magical moments from our vacation have occured in the strangest of places, walking along I-Drive, Eating at Ponderosa, sat outside our hotel waiting for a cab etc,.
