I have done my ATF fluid on my 2008 xB (WS Fluid) twice now, and doing it again in the next 5K miles... I did it at 17,500 and 30,000... I am currently at 45,000.
I like to do it every 15-20K, myself. It is half the work of an oil change, so why not. Pull the plug and refill. Easy as that.
I must say, my tranmission shifting is BUTTER smooth. I have yet to drive another car that felt as smooth as my xB. I don't know if that is from the constant changes (probably not! haha) or the quality of the AT, but I like the peace of mind that I am doing the upkeep on it. Fluid is still bright red and smells the same coming out as it does going in. Good sign.