I did the same thing yesterday. However, I wish now that I had chosen to select a google phone number. There are so many more options and features available if you do. Make sure you select carefully, you can't change your mind later! Have a look at the comparison here before you get your invite (if you've already requested one):
http://www.google.com/support/voice/...?answer=164819
BTW, it is true that if you're with Sprint, you shouldn't get charged for the conditional call forwarding. That change was made the third week of November, as it used to be .20 per minute.