I enjoyed it actually. I thought it was a cool building block to how Big Boss recruited his soldiers. However Kojima doesn't see it as canon or something.And Peace Walker I honestly thought was the most meh of the MGS games. I couldn't even finish it because it was always guiding you on a more small mission based thing and every boss was some tank or mech.To answer your question though, you can just straight into PW, HOWEVER if you just play Ground Zeroes, they sum it all up for you, because the story was really retarded (and that's saying something for a MGS game).
Really? I'm a massive MG fan and I thought Peace Walker was the most boring in the entire series. Only one I didn't finish.