Everyone has some great points. There was a loooooong thread on this a while back, look through the archives, but it comes down to what you plan to do with the car I think.
If you're a true race head, you'll probably take an approach like David's, which is better for the reasons he pointed out. You can put exactly what you want on the car for as much or less than the ACR package.
ACR Joe has a point as well, the ACR package itself, at least dollar-wise is worth it. The BBS wheels alone are around $2K each if I recall. Also, there were only about 115 ACRs built in each of 1999 and 2000. I have an all black ACR and I think there are about 92 all black ACRs total between '99 and '00 - so its fun to have a sort of 1 of 92 mentality about your car.
If you're buying used, like I did, I don't think you can beat the deal on an ACR. The price on used cars fluctuates so much due to mileage, age, etc, that its not hard (relatively) to find a used ACR for not much more than a comparable used GTS. When you look at the ACR package in that light, I don't know how you can resist. Even if you're a race head, you've got a good platform to start with at least, and its probably worth it at that point just for the wheels (assuming you like the wheels as much as I do - we ACR owners are very attatched to our BBSs, probably because we spend so much of our time cleaning them
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Hope that helps.
- Mark ('99 Black ACR)