My feelings are mixed on this issue. Like most here (and probably most metalheads, for that matter), music comes first, and I can acknowledge good music regardless of lyrical content. But in the final evaluation, I frequently do not separate the message from the music.
Do satanic or anti-Christian lyrics affect my tastes? No, not in the least. I was an avowed atheist before I got into metal, and I suppose it could be argued that those beliefs steered me towards a genre of music so accepting of them. But it ultimately musn't have mattered much, as very few of the first metal bands I got into were particularly satanic or religious at all. I was also much more of a guitar geek then than I am now, so I'm positive my pursuits were always more music-driven.
But that doesn't mean lyrical themes in general won't affect my feelings toward a band. It's just that the overwhelming majority of bands have lyrics towards which I'm either supportive or relatively apathetic, so it's not something I grapple with very often. I'm not a Christian, and I generally prefer not to hear lyrics with such a message in my music. But I recognize that there are bands like that (Antestor, Phlebotomized, Vomitorial Corpulence) that are unique or at least above average. But for most of these acts, there are many more with more "typical metal themes" whose music is as good or better, and overall I don't support or invest in Christian bands because of this.
But that certainly isn't a hard, fast rule, and my attitude has shifted over time, enough so that I now own a few Christian metal albums. I think I used to be like The Greys in that I felt Christian metal had a distinct sound. Ultimately, that was really just the power of suggestion and I don't feel this way at all anymore. Had I not been informed, I wouldn't have guessed the aforementioned bands' religious views just by the music. Moreover, these days I have more respect for those bands for going against the grain and standing up for their beliefs in a scene where espousing pro-God is often frowned-upon or ridiculed.