I bought a 2001 X5 3.0 about a year ago with 91k miles. Transmission had never been serviced. Three independent BMW shops gave me the same recommendation, to service the transmission as soon as possible, and then every 30k. That's what I did, and at least after my first 10k everything is still fine.

I have done some research on German websites (I am German) and read various used car buyers guides. Most of them mentioned that manual downshifts in the tiptronic mode WILL ruin an auto trans. Recommendation is to just leave it in auto all the time.

Hope this helps.

Dieter