I can't speak for the Google Voice App being fully integrated - I only use it for the voicemail feature. However I have a fully rooted Evo (Fresh Rom 3.5) and I still have access to the market and all the normal Google Android Apps. In fact I find that rooting gives me more features like free wireless tether and with Fresh 3.5 the power manager is integrated into the notification drop down (which I find really cool). Google Voice still works perfectly for voice mail.
As far as I know the only limitation is the one you have listed: it will void the warranty. However if something goes wrong there is a huge community over at XDA to help you out or you can unroot and see what happens when you bring your device in to Sprint ( I haven't had to do this yet so I can't speak on that part).
Rooting (for me) has also made my battery life WAY better and gotten rid of those Sprint apps that I don't want.
Hope this helps