It's worth it if you want near 15K RPM SCSI performance, at IDE pricing.... albeit still with IDE's much higher CPU utilization.
The 8MB cache version of the WD1000BB is far and away the fastest IDE drive available, but it's pricey. Still, if you want the highest performing IDE drive out there that'll crush anything else, while offering near 15K SCSI performance levels.
None: That's for desktop applications, for server usage it'll still be handily beaten by any modern SCSI drive as it can't offer the low access times SCSI can.