We have also used that one, twice in one trip. Try to get there 15 minutes prior to opening if at all possible. Our experience was very good.
However, we had a heck of a time getting our (normally very good) Anthem Blue Cross Blue Shield insurance to cover it. They did not consider the walk-in clinic an "emergency room", and since we did not have prior approval, they billed us in full. We just resolved it in November (I think, keeping fingers crossed) and our visit was in March.
Whatever you do, try to avoid the ER at Celebration Hospital. We went there with what we thought was our son's broken arm in 2003 (fell off the bed at AKL) and were not seen for 8 hours. Another 4 hours after that we finally emerged.
I also looked into having one of those house-call doctors come in to the house (this was in March -- same kid, different ailment, an ear infection) but the cost was way way way too prohibitive and my insurance would not cover it. I can't recall the name of the company, but there are at least 2 in the Orlando area.