I just got back from Vistana. I had a great time, but I can see why there are so many negative reviews. The resort is divided into 8 or 9 sections. It looks like two of the sections are fairly new or recently refurbished, Cascades and Lakes. Both of these sections are really nice and I think most of the favorable reviews reflect that. The other sections look like they were built in the 1970's, and from what I could see, don't look nearly as nice inside. Just older and more worn, but still pretty large and comfortable looking. I think a lot of exchangers in a Sheraton level resort probably were expecting it to be more upscale. The older sections are clearly gold crown quality if they were in say Hilton Head or the Outer Banks, but in Orlando the expectation is higher. Unfortunately, it's a crapshoot in what you get. I got Lakes and was very happy, but I met a few people at the pool who were not happy about their older units.
I'd still get a car if I'm staying at Vistana. Car rentals in Orlando are some of the cheapest around. I've never paid more than $200 for a week (taxes inc.). The resort grounds is fairly spread out, although all of the units are probably easy walking distance to one of the three major pools/activity areas. If you get Lakes, Fountains I, and Fountains II, you will be very far from some of the resort's amenities (probably too far for 5 and 6 year olds to walk) like the tennis courts, a not very nice minigolf course, the general store (think hotel gift shop with 7-11 attached), and the Pizza Hut (though they deliver). If you call a bell hop, they will shuttle you to the visitors center or the other areas of the resort, but expect to wait a half hour or more.
While the store and resort restaurants are OK, there is also a ton of chain family style restaurants, a Walmart, an outlet mall, and a couple of grocery stores within a few miles of the resort. If you had a car, you would have access to them, rather than rely so much on the on resort facilities.
The bus actually costs something like $9 or $10 pp for exchangers. So renting a car is only slightly more expensive than the bus and a cab roundtrip from the airport.
Another downside of the bus is that it is not flexible, it has several set times in the morning and afternoon. While it stops at the Magic Kingdom for all times, it does a rotation for all of the other parks. For example, the bus only goes to Animal Kingdom at 8am and Epcot at 11 am.
