RCX.NET can be found at...
I've tried using 2.5 and found I could not reliably control the RCX. Only 90% of commands were received.
Uninstalled SDK 2.5, rebooted, installed SDK 2.0, rebooted. From then on 100% of commands were received.
I'm using an RCX 1.0 (with 2.0 firmware installed) perhaps that also had something to do with the issue.
Hope this helps someone.
-g