I have recently done a test with IAA installed, with Intel UltraATA Strorage (old version of IAA) and with default WinXP dirvers.
I used HDTach and the cosbi benchmark. My resaults where very simmilar in each case, but IAA showed ~10% CPU usage, while the other ~5%. Read speeds in HDTach where also somehow better (and more consitent) without IAA. I haven't noticed any changes in boot up time or shut down time.
What I did notice was: diffrent HDD activity sounds (noise), maybe because of some acustic managment.
And more importantly my HDD makes a diffrent sound when it shuts down with IAA (louder and longer click) and sometimes when resetting Windows IAA caused the HDD to spin down and then back up. I am not using it now.