To make free calls using GV, you need a Gizmo5/SipGate/any other SIP account, and the Sipdroid app on your phone. While it's not fully reliable yet, it does work every now n then. You can also set up a SipSorcery account and with the right scripting, can dial directly from your phone without having to initiate through the GV website.
It works over WiFi as well as 3G.. but as I said.. it's not very reliable atm.
Here's a full tutorial on the matter:
Unlimited Free Calling with Google Voice - Google Docs
As for GV doing voicemail greetings like Youmail... Youmail does it for EVERY contact.. not groups. It even greets them by name according to your contact entry. If I actually used my GV number most of the time I might try it's VM, however since I still use my verizon number.. it was easier to just have it forward to youmail. It even has a free visual voicemail styled app for the phone, whereas I don't believe GV does. I could be wrong though, but I didn't see one when I last messed with it.