We've rented houses in Kissimmee either 4 times I think. All of the houses were in the Lindfields development on 192 West, about a 7-10 minute ride from Disney property. I believe the company we used was Styles Estates, though they were called something else when we first used them.
Our experiences have been flawless every time. Anyone traveling with a large family or more than one couple/family traveling together couldn't do better. The houses are very spacious, clean, and well-maintained. I don't care what hotel you stay in, there is no comparison to putting the kids to bed in their own rooms, closing the doors, and relaxing by the pool or in the living room (often with a VCR). And if you are splitting the cost with others, it ends up cheaper than all but the lowest end hotels. For example, we once rented a 4-bedroom house with my mother and mother-in-law. They each had a bedroom, my wife and I had one and my daughter had one. We split the cost, about $120 per nite, 3 ways so it only cost us $40 per nite!
I absolutely recommend it. And if you have too many people to fit comfortably in one house, rent two that are close by. That is easily doable down there and we have done that as well.
Steve