I just ran into the same issue you are One_Hertz.

I picked up an X25-M and tried running it on an Adaptec 31605 and it benched like crap. Read/Write speeds were all over the place. With cache on, it did horrible, with cache off, it did not as horrible.

Plugged it into the ICH10R on my P5Q, and it was above intel specs. Ran great-

Needless to say, Adaptec needs to catch up with their firmware. It's actually kind of nice to see an onboard solution work so well. I am selling my Adaptec 31605 and 2x Cheetah 15k5 SAS drives to make up some of the of cost of the X25-M drives..