One feature in the Google Voice (which was a part of GrandCentral as well) that is very useful is that the ability to mark a caller as spam, just like you do in your Gmail. Once tagged no more future calls from that phone to you. The caller will not be in your message list (Voicemail) but as I said, just like in Gmail, will go straight to your spam folder.
This will only affect your account and the blocked numbers are not shared among other Google Voice users.
But there are exceptions to the private list of spammers. Google Voice will block reported and confirmed phone spammers, like automated dialers that call you everyday.
One I really like to get blocked is the annoying calls trying to con you into buying a vehicle warranty. Previously I could only play along with them if I had time. Now I can block them straight away!
According to Google Voice blog post bellow, there are tens of thousands spam phone calls are blocked, thanks to Google Voice.
Google Voice Blog: Helping you fight Phone Spam
Wednesday, April 01, 2009