When I got my droid, I was really pumped about being able to use Google Voice exclusively. The features of Google Voice are really exciting: basically a router and firewall service for your phone with voicemail transcription, and a permanent record of all of your voicemails and SMSes. I'm sad to report, however, that I've stopped using my Google Voice number until further notice. Let me explain:
First, the experience of switching over to a new phone number was pretty painful, and painful in a way that they could easily have fixed. If they would let you do a one-time whole-phonebook text, which you can't (you can send to multiple recipients now, but limited to 5 people at a time), made it very difficult to tell everyone "this is my new number". It's not like I really want to call everyone in my phonebook or send them each individual text messages with my new number.
Second, I had a lot of trouble connecting calls the Google voice. There were a lot of times when I would try to make a call via Google voice, and it would never connect, even though my phone was reporting a data connection, then I would try to make a normal call and it would work.
Third, and most importantly, the lack of push SMS messaging rendered a much-loved communication medium useless. I simply can't wait until the next time the app polls for new messages. Often, the app would close without notifying me and I would go an entire day without getting the text messages; wondering why, I would start the app and receive 28 text messages all at once. I'm aware that there are workarounds for this involving e-mails that ARE pushed to the phone, and I can also just use Google talk, but they just don't seem very good.
I'm sure that people will tell me that Google Voice is a free service and I should be happy with it as is, but I maintain that Google will eventually be inserting ads so I think I am allowed to be picky.
Note that if they add push text messaging, I will immediately start using it again. Are you listening, Google? :)