Does google voice ever cost anything?
You can cut your texting plan because Google voice sends texts as data however you cannot send or receive mms through Google voice. Search the forums there are many threads on this already.
As I understand it google voice DOES use minutes. It is not like skype. Instead, effectively, you call google, then google connects you through its servers to the other person. Likewise when someone calls your GV number, they connect to google then google "calls" you. Of course it is all totally transparent. That's why gv can do things like make all your phones ring or route certain people to certain phone numbers. Also why its cheaper for international - you are making a domestic call to google then google routes you to the international person. But make no mistake, the link from the droid to google is through a normal cell call and DOES use minutes.