Never really paid much attention to Tool though they fall in the heavy prog metal area and I knew a few people who dug them some years back.
Prog is my main interest so I just listened to Lateralus (which I heard when it came out ) 10,000 Days, and Fear Inoculum. My ears are still ringing.

I have to say I like the strange lyrics and references to occult/spiritual matters and even ufos/aliens. The artwork is also very interesting and I am familiar with Alex Grey being into surrealism and I know he does most of their art covers.
As far as the music, I like many of their tracks though at times there is a sameness to the song structures and they blend into each other which might be what they are aiming for.
I currently dont own any Tool but I am going to look for used cds at my favorite record shop in my town next time.
THis was interesting being one of their weirder quieter ones about a guy who had a traumatic encounter with aliens....?....which blends into the next song with a similar alien theme but more intense.