I'd might worth it for the next few years, but new standards will probably be established then. Like it was said before, there isn't much of an increase when you switch to ATA/100.
The bottleneck here is the Hard disk. The fastest IDE hard disk cannot reach a burst read peak of 60mb/s, for now. So the ATA/100 is sort of overkill. Though, I believe that 60mb/s is pretty close to the limit offered by UDMA/66 (66mb/s), so we'll probably see a use for ATA/100 pretty soon. But, do not expect a 50% increase of data transfer rate.
I wouldn't say there was a 50% improvement, but there is definately an improvement! The problem with IDE drives is that they have a large processing overhead (SCSI gets over this by having better controller cards), so you loose a huge chunk of bandwidth.
So even though the drives themselves can't transfer at 100Mb/s the ATA100 standard means that the drives do nearly reach there peak transfer rates. Where as ATA66 is limited by bandwidth.
For the ultimate IDE performance get a Promise Raid card and two UDMA100 hard disks