Right, Slipknot are one of the most interesting metal bands of the 21st centuty (if you forget the gimmicky masks and the dodgy lyrics).
First you have 'Mate Feed Kill Repeat', which is a great exercise in whimsical, jazzy thrash metal.
Then Ross Robinson re-invents them with the raw, aggressive, weird 'Slipknot'. Seriously, if they better 'Sic' then I'll proclaim them the best band ever.
'Iowa' isn't bad, but is mainly filled up with derivative thrash metal. Nothing new there. Still, kudos for it to getting to no.1 in the album charts - the first song is called 'People = shit'! Crazy.
Then you have the definitive Slipknot album, 'Vol.3', which marks a progression in their sound while retaining the energy from 'Slipknot'. The marching band drumming in 'The Blister Exists' is a fantastic touch.
New album is coming in 2008 apparently. Hurrah.