artemis entreri has it right: while Tool's music doesn't look too impressive on paper, and they don't play at inhuman speed, the tone and texture of their sound is almost impossible to duplicate, and this is what makes them masters of their instruments. But in a more general sense, they are masters of music (and this is a very important distinction: it doesn't matter if you can play like Jimi Hendrix if you write like N'sync).

Tool is probably the closest thing in modern rock to classical music. Ever listen to a fugue? I'm sure you have; if you aren't familiar with the term, go fix that right away. Now listen to Forty-six & 2. Pay particular attention to the solo/breakdown bit, the last two minutes of the song (note: I've been trying on-and-off for months to tab this bit correctly and completely for all instruments. It's absolutely impossible.) It is - make no mistake - a modern fugue (albeit shorter and less complex than something Bach would've written). Listen - there is the basic theme, and then we hear it transformed, played over itself delayed, played with a different rhythm, different emphasis, moved up an octave, etc etc. It may not conform to all the rules of a fugue (I don't know them so I couldn't say) but it retains the basic concept.

Tool's music, unlike that of most bands, is orchestrated; it is thought out. The vast majority of bands in modern rock follow the basic pattern: think up a couple chord progressions (hey, lets use power chords, they're easy), think up a tune for the verse and one for the chorus, insert cliched and hackneyed lyrics here and viola we have a song. If they're really lazy, they can even use the 12-bar blues and dont even have to think up their own progression! (Note: a lot of very good music is in 12-bar blues (lots of Stevie Ray Vaughn, for starters); their strengths lie in the incredible soloing. There's also a lot of shitty, generic 12-bar blues out there.) Tool doesn't play like that. In fact, I can't think of a Tool song where a guitar actually strums a chord; they prefer to let the chord be expressed through the amalgamation of what all the instruments are playing. A similar style is often employed in classical music, as many instruments can only make one note at a time.

It is true that some of Tool's music, particularly pre-Lateralus, is much less complex than Forty-Six & 2. Opiate, their freshman album, is just barely outside the realm of average alternative music. Undertow and ├ćnima are better, but still contain a few songs that have little instrumental complexity (although generally these songs are balanced with lyrical complexity, like, say Hooker with a Penis). But it is worth noting that pre-Lateralus, Tool refused to write music that they couldn't play live exactly as they did in the studio - so no layering. Lateralus, however, brought with it the wonderful world of studio enhancement (though it STILL sounds exactly the same live; I am amazed. But anyway), and most of the songs on Lateralus have a high degree of intrumental complexity. Lateralis, The Grudge, Disposition, Reflection, are all very similar to Forty-Six & 2 in this respect.

There is a further degree of complexity to Tool's music, however - the scales they use. Anyone familiar with Indian Carnatic music reading this? Listen to Sober, off Undertow. Try and discern the raga. It's Revathi. Listen to Reflection. You'll hear Sindhu Bhairavi (and listen to the drum in Disposition, its a tabla!). Tool uses scales (and, indeed, other musical concepts) from all over the world in their music. They use Indian and Middle Eastern ragas quite frequently, as well from the Far East and other places around the world.

Tool, taken in pieces, is not difficult. A guitarist with a few months of practice could probably play the main riff from Forty-Six & 2. But it takes true musical mastery to compose the way they do, and to play so perfectly in step with each other as they do. And it takes a true understanding of music to bring together concepts from all different parts of the world and make really good music out of it.