Pretty much what's been said; a good lyric works on it's own. Reading them without hearing the song is pretty much the only way of judging lyrics - occasionally, a really talented singer can make even the crappiest lyrics sound great. For an example, try listening to Axel Rudi Pell's "Come Back To Me". The lyrics are downright horrible, a mix of over-used clichés, sappy phrases and things that just makes no sense. But Johnny Gioeli somehow manages to make it sound good.

As for good lyricists... Leonard Cohen, Nick Cave, John Oliva (of Savatage fame). I have a soft spot for David Defeis (Virgin Steele) as well, though his style ususally requires quite a lot of background knowledge to "get". Especially "The House of Atreus" 1 and 2 gains a *lot* if you are familiar with old Greek legends and imagery.