I love the 540i. Solid, well made, and fast.
They do seem to get more than their fair share of vacuum leaks. Fairly easy to fix, though, just a nuisance to find.
Another area is the small hose from the fuel pressure regulator to the intake - tends to collapse with age.
Both of these problems can give weird symptoms. Some have mistaken these easily fixed problems for the infamous "Nikasil" problem.
<img src='http://bmw-540i.com/images/540i/540i.jpg' border='0' alt='user posted image' />
<a href='http://www.bmw-540i.com' target='_blank'>My BMW 540i website</a>