I almost always write the music first. I've tried writing lyrics before music, and for me it comes out sounding...well, like I forced music to fit the lyrics. When I write the music first, I can focus on the groove and find the words that fit the musical impact. And since I sing my own songs, I avoid certain words because they sound stupid when I sing them.
When I see someone post lyrics here, I always refrain from giving comment, because there is no way for me to personally appreciate what someone has written from a lyrical point of view without the music behind it. Without music, it's just poetry.