Player buy-in is important. Before I use a particular setting or genre, I want my players to both articulate and agree on their goals and the types of characters they wish to play. Do they want to be rebels fighting an evil king, dungeon delvers seeking fame and riches, or pirates on the high...
The good:
The sourcebooks are amazing. They make great reading and are useful for use in other systems. For example, GURPS space had charts and tables that I used for a savage world campaign. I used GURPS tech levels for a low fantasy game I ran, where some nations had gunpowder and others...