Tool is anything but aimless. They do have some progressive roots and influences. Musically speaking, their riffs are relatively simple, but it's the fusion of the guitar, bass, drums and vocals that create an overall impression. Nothing really takes centre stage at any time. They are, each one of them fantastic musicians in their own right. The way a Tool riff repeats over and over is VERY similar to Opeth. The difference is just the style of the riff.. Tool is more about chords than notes and melodies. The vocals take the melody in Tool music, where the notes carry as far in Opeth.
But Tool is every bit as aimless as Opeth is, that is to say not at all. All progressions are carefully metered and coordinated, nothing is random or uncertain.
Their live show is something completely different - and not something by which to judge the music on an album. I havn't seen Tool live yet, but I did see A Perfect Circle, whose vocalist is the same as Tool's.. and his stage persona is essentially the same (but he wears a long wig for APC, he's bald for Tool). He's flamboyant, usually wearing little to nothing, and that's how he performs. One APC show, he was ill and sang most of the show with his back to the audience - and people still loved it because the music is the star. That reminds me a lot of Opeth: not big rock figureheads, no pretentious egos - the music is the star, not the musician. They are merely sharing what they've created.
Tool is played as much as Opeth in my apartment. Easily tied as my favourite band. Many of Tool's fans (ones who actually do care about the integrity and emotion) are very similar to the people on this board, with equally complex thoughts and verbosity(neologism?). I identify with both, so I'm happy where I am.