I have been using google voice (GV) for 3 weeks now. It has it's pluses and minuses. There are two options when setting up your GV accont. You can 1) use your existing cell phone number or 2) pick a new GV number. To be able to use all the features of GV you are going to have to go with option 2.
Here is a link to compare the two options:
Using a Google number vs. a non-Google number : Basics - Google Voice Help
Things you need to know:
1) currently GV does not support mms. If you don't do video or pic txting this isn't an issue then.
2) you do not get sms (txt) notifications when the screen is off. (GV does not currently use push for their sms notification system. They are working on migrating over to a push notification system but I don't know when that will be)
Those are the two major issues that I have come across. I was getting sms notifications on every txt message I sent but after updating to the newest version of GV that issue has seemed to of gone away.
Unless you tell the default sms program (messages) to not notify you you will get double notifications for sms messages. This really isn't annoying as long as you don't mind having multiple icons in the status bar plus the battery issue with that program I believe has been fixed. Though I would prob recommend not disabling the notifications that way you will be able to get notifications while the screen is off.
I am not sure how Sprint views GV calls (landline or mobile). This becomes important if you have an unlimited calling to any mobile number plan being that your mins to landlines are limited. One user told me that Sprint treats GV calls as mobile calls so they shouldn't go against your landline mins, though I am waiting to hear back from him to confirm this. You might want to call sprint yourself to confirm this independently (if you do please let me know what they say)
I can't report any problems making or receiving calls/txt messages. I haven't quite figured out how to get the pics to show up in the call screen. Right now my contacts pull their pics from either facebook or gmail but I am getting the stock green android on the calling screen.
I was able to make an international call successfully to South America. Though txting the same number did not go through. International txting is not officially support so it is hit or miss as to whether or not it works.
GV integrates nicely into the phone. I have it set as the default program for calling and txting so it launches for both of those activities by it self.
if you have any other questions let me know.