avsforum.com is the best one I've found so far. It has plenty of traffic and plenty of people to answer questions. Just don't ask them if you should get a tivo.
I had a ReplayTv untill about two months ago when it was stolen, and it was great. It's scheduling isn't as good as a Tivo, but most everything else is about the same. The ones with ethernet allow you to (unnoficially) stream to your computer, which is really the holy grail of PVRs. (Well, that and HDTV, but no one does that yet). If you havn't had a PVR before, you'll love it.
Matthew