The last few parks we've stayed at that offered WiFi had decent enough service. Stone Mountain Park in GA has a nice setup. Their speed isn't great, but it is good enough. Their system also monitors bandwidth usage and restricts usage to bandwidth hogs.
Our seasonal park has pretty decent WiFi too. Their backbone is pretty stout, down by the office, I can get speeds of 75Mbps down and up (tested with Speedtest.net). They use a Verizon FiOS service. At the camp site, I get about 15 - 20 Mbps down and up. We are 3 rows back from the lake and the office is on the other side of the lake. From my site, I can find 5 access points on a WiFi analyzer.
-Michael