SCSI IS ROBUST:

   One thing most users don't know is that there are different standards for calculating Mean Time Between Failures (MTBF) for hard drives. The industry standards for E/IDE (ATAPI) is based on a 8.33 hour per day, 5 day-per-week usage (think in terms of heat generated & cooling times -- a 42 hour week). SCSI drives on the other hand are considered the defacto standard for mission-critical usage by drive manufacturers; the MTBF for SCSI drives are calculated based on a 24/7 heavy-duty usage as would occur in a server. As a result, SCSI drives (and SCSI in general as a set of communications standards) are more robust. Furthermore, because of their intended usage (servers, mission-critical applications), SCSI drives are designed to have speed advantages over E/IDE drives in real-world usage.
   There's a reason the SCSI was the standard on the mac for over a decade: speed & reliability. There's also a reason that it isn't the standard any more: cost (if you visit the Apple Store online, you can opt to have your new unit built with SCSI instead of ATA/EIDE, but it will add a steep premium to the cost).

ATA stands for ATAPI, also known as IDE and E/IDE in industry parlance. The names are interchangeable in general and when I refer to one, I am referring to any of the three.

 
SCSI IS FAST:

   Most power users understand that SCSI is faster than ATA, but tend to use raw numbers to compare data transfer speeds. While IDE transfers at 33Mb/sec, and E/IDE at speeds of 66Mb/sec, 100Mb/sec (and a new revision supports 160Mb/sec transfers, although it's not built-in on any mac built to date), SCSI can transfer at speeds of 160Mb/sec per channel and can support multiple channels simultaneously...
   But that's far from being the end of the speed difference. IDE does not support blind transfers, request optimization, request queuing or stacking; SCSI supports all of these to give it a real-world speed benefit over IDE/EIDE/ATA (ATAPI). So what are these issues, that they provide a real-world speed benefit?
 
Blind Transfers - if you are copying something from a SCSI CD drive to a SCSI hard drive, the SCSI controller tells the CD drive to target the hard drive and start reading at position X. After that, the controller can step out of the transaction as the CD streams the data across the SCSI chain to the hard drive, straight from one to the other.
  The same process under IDE would require the controller to perform substantially more steps because it doesn't support blind transfers. First, the ATA controller would have to request the attention of the entire chain, effectively locking out any other requests; then it would get the data from the CD and move it into memory, close that request, start a new request to the hard drive, and then write from the memory to the hard drive. Repeat in small chunks until the whole file is moved. As you might imagine, this consumes more than twice the time, even at the same transfer speed!
 
Request Queuing or Stacking - SCSI drives are capable of taking multiple requests to read and/or write data at once. Let's say you open 5 small files -- under SCSI, the request for all five would be sent from the controller to the drive(s) as single request, and the card would then disengage from the SCSI chain and wait for the data from the drives to come flowing back. And because multiple requests are possible, the previous example of transferring data from a CD to a hard drive could continue during the time it takes the hard drive to grab the five files and return them.
   IDE/EIDE/ATA on the other hand, only supports a single request at a time, and the controller must wait for the data to be returned before releasing the connection. For five files, the IDE/ATA controller would have to make 5 separate calls, each with it's own session overhead; the CD to hard drive transfer mentioned previously would have to complete before the IDE controller could even begin to grab the file files (at least on the hardware level -- software in the background can make it appear to be happening simultaneously, but in reality, one copy process is made to wait). Again, SCSI is the clear speed winner at the same transfer speed.
 
Request Optimization - As noted in the previous paragraph, SCSI drives can store multiple requests to read and/or write data. On many SCSI drives (on virtually all SCSI drives built since 1991 or '92), there is on-board request optimization. On-board request optimization means that the drive can optimize the order in which it reads (or writes, or both) the data requested -- in whatever the fastest order is that it can fulfill all the requests. In our example, we asked for five files; the SCSI drive can figure out that file 4 is on the way to file 1, and optimizes the getting of data to reduce the total time required to grab all the files. IDE does not support multiple requests in hardware, and thus would grab file 1, complete the transaction, then grab file 2, complete the transaction, etc (you get the picture).

  If your work is 100% surfing the net and word processing (not DTP), then the speed differences between SCSI and ATA are probably not significant to you. However, if you spend your time working on photo-manipulation or graphics, DTP, sound-editing, video-editing/post-edit, run databases (which are always inherently hard drive intensive), web design, programming/development, or run a RIP, then you need to be using the fastest SCSI cards & drives you can afford. SCSI has a real-world benefit for those who save & access large files, and those who save & access large numbers of small files.
   The speed differences are particularly visible to users of FileMaker and Oracle databases (up to 20x faster in the real world!).
   Furthermore, if your boot drive is a SCSI drive, then all the system access calls (which are made by every program) suddenly become faster, making this SCSI one of the easiest ways to truly increase your machine's speed and thus increase your own.