We've used other agencies and also rented from private individuals, but All Star Vacation Homes are THE best for service and condition of the homes.
We have a large group, always, and we rent one of their larger 5,6,7 bedroom homes with pool and hot tub. They even have a grocery delivery service that brings food and stocks it in pantry and fridge. We've stayed in homes with game rooms and heated pools and the kids beg to go home early from the parks so they can play and swim.
In April, paying to heat the pool is worth the extra money; the water can be a bit chilly. The houses with pools have 'kiddie alarms' so that's a plus for younger kids. Any time there's a problem, it's fixed right away and check-in is simple. Each group gets their own house key, etc...
We love to stay on site, too, have been going to Disney for many years. But the house is a nice, relaxing change after a busy day of crowds in the parks. It's a much more relaxing vacation, I think. The houses are just a hop, skip and jump from Disney; not a bad drive at all. Sure beats the campground (where we've also stayed from time to time), especially if you're tent camping.