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.
Edit: I can't comment on eldarerathis's post otherwise this would have gone there: I had forgotten about hardware limitations of using AOSP Roms like CM. eldarerathis is correct though that with certain roms HDMI and 4G won't work. When I had the HTC Hero and first loaded CM I think the Camera didn't work but in time this was fixed and this is why I have avoided the CM Rom so far for my Evo.
If you want the HTC Sense experience though don't go with CM or AOSP roms - these are essentially stock Android.
Hope this helps