Ok, I've been running A64 Tweaker for the past couple days.
Pros:
Well, the performance boost is nice, why else would I be trying to use this program in the first place.

Cons:
The first thing I notice is that the settings made by A64 Tweaker are not retained after resuming from Hibernation. I haven't tried Stand By yet. So every time I power up my system from a hibernated state, I have to re-apply the settings.
It's also frooze my system once, the instant I clicked on the "Set" button.
What I don't know is why the system freezes. Is it a glitch in the code, or could it be that the AMD PowerNow system was throttling the CPU when I hit the set button. I don't know.
When it works, it is nice. Everything feels a bit more responsive.
So far, I can't find any negative side affects from running the system at DDR-400 speeds. But I've also got RAM that is rated to run at DDR-400 speeds. I don't think I would want to try it with the stock RAM.