I have had my WS-2000 since Aug. and don't pay anything for ambientweather.net. Their service has been reliable and has only been down one time that I noticed during this 4 month period. During that outage, I wasn't able to access the web site for a couple of hours, but they did continue to accept the data from my WS-2000, so no data was lost.
I also use Weather Underground. There are a couple of things I like better about WU than Ambient. WU updates every few seconds, while AmbientWeather updates every 2 minutes. It also seems to me that WU does a better job of averaging wind speed and direction that AmbientWeather. The graphs on WU show more consistent and smooth wind graphs than Ambient which shows my wind graphs as kind of all over the place. Maybe that is because of the more frequent updates on WU.
Overall though, I have been pleased with AmbientWeather.net.