my car is an '06 550 with 53k on the clock.
I was doing my brake flush this weekend and noticed these sensors were both weeping with oil residue reaching the bottom of the block.
Quick note: You can only see these clearly when the cowling and air filters are removed like for doing your brake flush.
So how hard are they to replace and what are they?

 

Those are the eccentric shaft sensor electrical connectors. The two screw plate has an integrated rubber seal to seal the ESS connector stem. The rubber hardens and then leaks.

 

Thanks for that!
Just found the sensor gasket seals from a BMW dealership on amazon for $15.00 for 2 with free shipping. Phewww going to be the cheapest DIY yet. I noticed in the dealers small writing it was explaining only a 2 year warranty or 24,000 miles.
this may be a place to check if your having oil loss or leaks.