It seems that as currently configured, Google Voice does not allow more than one of their numbers to be forwarded to any particular phone number. That means that if I have several visitors staying with me, no more than one of them can have his calls forwarded to my home number via Google Voice.
A second restriction is one I described in an earlier post: that call-forwarding cannot cross national boundaries. I can see that that prevents people from getting around international phone charges, but the rule applies even between the US and Canada, and at the moment US-Canada phone calls are subject to no charges on Google Voice. So in this respect call-forwarding is actually more restricted than ordinary phone service is.
I ran into both of these problems when trying to set up Google Voice as a way of avoiding long-distance charges for someone without computer or cell-phone skills.