On my test server I have installed xcache, eAccelerator and memcache..
I cannot come to any useful conclusions, yet, though, because my test forum is still in 3.0.5 (I copied the modules from 3.0.6 SVN and enabled them in config.php)
So, although I admit that my test forum has little to null traffic I can't say that I've seen any speed changes, because I understand that the performance increase is not only in the acm modules, but in the code, too.....
My live server does not have any accelerators installed. I will have to persuade my host to install one and try to notice any differences..
I'm actually very interested in seeing whether there is a noticeable performance improvement, since I have a relatively big forum and use shared hosting. I have no performance issues ATM but it is always nice to realize if I can avoid the Dedicated server cost for a while longer...
Anyways, I would be happy to see any comparison data here from anyone that has tested these acm modules..