alot depends on the controller you use.
I've got some experience with 3Ware controllers, the ones used in The Dolf's link.
The are really fast (I get 150mb/s write and 160mb/s read in ATTO Disk Benchmark) combined with a large number of disks (I use 5*120GB 7200 rpm by WD)
Making a 600GB RAID rig with IDE had cost me 500 for the controller and 130/disk
If you want the same capacity with SCSI RAID u'll have to spend an enormous amount of money then, but that is not the problem for companys who want fast and reliable storage
SCSI is faster, is more reliable than IDE but the price I quite a lot higher
Conclusion: you are not a richass and want a lot of fast storage: good ide raid controller and ide disks
you are a company that needs reliable and super-fast storage: scsi raid
(addendum: using raid 5 with 1 or 2 hotspares makes IDE RAID almost just as reliable as SCSI RAID; if you replace the broken disks immediately that is so you cant run out of spares

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