Synergy isn't always useful in all situations. Whereas I agree it is useful if you have one machine that will always be on whenever the others are, then yes it is useful. However if you don't have that, it's pretty useless.
I find synergy can be a bit buggy for gaming. In a lot of games the input will suddenly be split over both systems as synergy gets confused for some reason. Mouse still goes to the client but all keyboard input goes to the server and the only way to fix it is to plug an external keyboard into the client, kill synergy and start up synergy again. Very annoying - particularly in games like Dark Messiah or Bioshock. :|