Very, very important, for many reasons. Lyrics can often heighten or decrease the experience of listening to a song for me.
For example, "These Walls" if it was simply about some vampire story like Forsaken, I'd simply consider it a very catchy song, but because the lyrics are able to convey such real and personal emotions, I find listening to the song to be a powerful experience.
Vice versa, "The Count of Tuscany" if it presented lyrics of the same literary caliber of "A Change of Seasons" the song would be a thing to behold, but because it has such extremely poor lyrics, I find myself very detached from the song despite its level of musical excellence.
Aside from all of that, I consider myself a writer, and someone who appreciates the literary ability to give insight to different aspects of life, so when lyrics are written very tastefully, I find myself not only able to enjoy the lyrics, but I can be inspired by them, and they can change my perspective on things.
I also love when a musician has the capability of writing lyrics grounded in well crafted metaphors, which allow the listener to interpret their own meaning. Usage of good imagery and tasteful vague writing can really provoke a very poetic experience.