I was asked to check into it for work as an experiment. I have had it for four (4) months now. Prior to getting my HTC Eris, I only used it at work as it was difficult to have to call in on my cell phone to use previously on my Palm Treo 700W.
I exported my work contacts from Outlook to my Google account (follow the instructions on Google Voice) and with a simple click on my computer on the contact; it calls my phone at work (or which ever phone I want it to call me on) and connects me to the contact. When people call me on my Google number that are in my contacts (I keep most there for this reason) I can tell it where to contact me, and it will ring as many phone numbers as I tell it. So for work contacts, it will ring my cell and office phone and I can answer either one. For personal contacts, it rings my cell and home phone. For my kids, it rings all of my numbers. I can even switch phones on the fly - say I'm in the office talking to a client and need to leave, I click the button on the phone and it transfers the call to my cell phone and they hear nothing on their end and I can head to my next meeting, call it a day, or head to lunch without interrupting them (again follow the directions on the Google Voice website).
As for use on the HTC Eris, I have Google Voice set as my default for all calls and anyone getting a call from me sees my Google phone number, not my cell number. On my phone you do not even know I use it. As for if I am still using minutes on Verizon, I am not sure, but would say yes, as you are still using the network to place the call? I have not heard any complaints on quality of the calls, but I have had probably 4 or 5 occasions when I had some difficulty placing a call - system error. I guess I have never noticed any delay issues on the calls either.
As for Voicemail, you can set up different voicemail messages for different people or groups easily. Coworkers (separate group set up) get my work voicemail, personal get another, and clients another, I merely have to have them in my contacts on Google Voice. It's all very simple.
I like it because I give out one number to everyone, they can always reach me without calling several numbers.
If one of your coworkers has Google Voice, he can send out up to three (3) invites through his account?
Good Luck.