I have pretty much given up trying to actually hit balls from my ball machine using the pseudo-SweetSpotter I built but I've found it is a great tool for warming up before a match. I'd compare it to swinging a weighted baseball bat while in the on-deck circle.
It really helps me focus on getting my swing speed up and since it has a tennis racquet handle it feels more realistic to hold. I've been keeping it in my bag and swinging it before matches and it seems to help.
When should you serve and volley in doubles? Only when you want to win.