A copper spacer or shim as they're called keeps your heatsink from putting uneven pressure on your AMD CPU core and chipping the edges of it. IMHO it's great insurance for around $9.00. You might want to consider a non-conductive one though so theres no chance of it shorting out any of the bridges.
<<The conclusion was they interfere with cooling>>
I saw two reviews that proved otherwise with coolerguys.com's non-conductive shims. AMD recommends 20 lbs of force on these cores and that's a lot. And with these shims, you can be sure that the chips wont crack if you make a mistake and apply uneven force. If you never put on AMD heatsinks before or you plan to take it on and off frequently, it's great investment.
Yeah, I've read that as well. If my memory serves correctly, I believe I also saw an article on the copper ones that says Cu-shims enhance heat transfer. I'm not so sure about that myself. For what it's worth, I'd go with the non-conductive shims, regardless of whether or not they enhance cooling. One Celcius degree isn't much to ask when it comes to CPU insurance.
There is no substitute for care, but I'd prefer to err on the side of safety.
I've read that the copper shim creates more heat with the Intel FC-PGA C-II and P-III chips, but might help cooling on other chips. My personal experience with a copper shim and a P-III 700 was that it hurts cooling by 1 -2 degrees. Now I don't know about the AMD chips.
They do, any shim will, but it is only 1-2 degree C. This is not a huge difference. For $10, you CAN'T get a better insurance policy. I run a nonconductve shim(copper ones are bad, the conduct a whole lot more heat and radiate it down to the cpu) on both my celey and a duron 700. They keep peace of mind, especially the Duron with a Fop38 HSF.