Just to share my experience... I finally did the drain and fill (minus pan drop/filter replacement) on my 328i for the first time today, just shy of 124000 miles. Drained out about 3.5 qts and took about 4 qts going back in. The old fluid was a very dark brown, with a very slight hint of red, but didn't smell burnt.
The result: incredible. It shifts so much smoother, I don't even feel any jerks in the shifts anymore. Drove it around quite a bit and haven't noticed any abnormalities or slipping either. Absolutely well worth it.
I am planning to do a second drain and fill after a few more weeks of driving so I can get more of the old fluid out, this time with a complete pan drop & filter/gasket replacement.