I've just stuck to the BMW service intervals. I know a lot of people change the oil and filter more often, but none of my cars has done more than 8,000 within the 2 year time interval so that seems frequent enough. I did have the transmission fluid changed in my 2nd E86 at 82k miles because I don't like the idea of it being filled for life. My first one had only done 62k when I sold it so fresh fluid didn't seem necessary.
My first one needed a new electric water pump at 60k miles and my second suffered from corrosion on the rear brake pipes at 84k and 11 years of age, so I had them replaced in Kunifer.
It's worth looking out for oil leaks from the cam-cover and oil filter housing as the gaskets can get brittle over time but other than that it's just been normal consumables.
I've restricted this to my 3.0Sis as the M requires far more TLC (and cost)!