The way I see it, the instruments and the melody are the first determiner of how good a song is, and great lyrics are a bonus.
Of course, sometimes when I learn what the lyrics are it makes the album less enjoyable because you start focusing on the lyrics instead of the sound. I had this experience with Jens Lekman. I saw him in concert, and many times he would preamble his songs with an explanation of the lyrics. So now when I hear "I'll be your boyfriend so you can keep seeing your girlfriend", I'm not focusing on the great melody. I'm focusing on the cuteness of the story.
But there's also the other side, like The National. If the lyrics were anything other than "Stay inside till somebody finds us, do whatever the TV tells us..." the song would be less enjoyable.
A lot of artists, of course, use the lyrics to show off how weird they think they are. People such as Brian Eno, Scott Walker, Frank Zappa, Captain Beefheart. It's almost impossible to imagine how you might listen to their music if the lyrics weren't so odd.
(Just try listening to 'We're Only In It For The Money' with different lyrics and say it's the same music!)