So is Guitar Hero: Greatest Hits worth buying? It depends. If you've at least not played either of the first Guitar Hero games, then yes. The majority of the songs come from those two games so a large chunk of the game will be completely new to you. If you've been a Guitar Hero player since the beginning, it's really just a case of whether you want to go through songs you've done before.
It would still be a good idea considering every song has been cleaned up, drums and vocals added and all are now Master Tracks unlike many of the original games' songs. So simply put, if you want to experience Guitar Hero songs of game gone by, buy it. If you've had enough of the old games, maybe you should wait until Guitar Hero 5 in September.