Alcohol 120% or 52% users should know that there is slight incompatibility during SP2 install which was noticed back in RC2 but apparently the problem's not in MS's end...
Anyway, SP2 cannot copy atapi.sys when you have Alcohol installed, regardless of it being currently running or not. You need to go to \Windows\System32\drivers and delete atapi.sys, then hit retry until SP2 continues installation. SP2 creates a new atapi.sys which is the same as the one you deleted, at least sizewise.
I've been testing both RC1 and RC2, I can say that SP2 is pretty good. Definitely a recommended install regardless of how good your system is. Like with any software out there, installing it may screw something. But MS had quite a long testing period and such occurences should be minimal at least.
It is true that SP2 may break some apps and games but patches will be out soon. And people who are annnoyed they need to do some setting changes after installing SP2 are just bad whiners. Big deal, just do the setting changes and be done with it. Everything can be turned off if necessary, including XP firewall and IE pop-up blocker.