Other than the 'usual maintennance' items, and well documented and discussed to death topics regarding the 540i, I've had very few problems; mostly normal wear and tear.

I bought my 540i from a stealer as a "Certified Preowned' with 45xxx miles. I'm approaching 149,000. The worst thing that happeend was the infamous Cyclonic Valve (PCV) failure. and tranny noise, which got me a free 5HP30 valve body upgrade. I've had 2 leakdown tests and passed both.
I can pile 3 windsurfers and gear on the roof rack and cruise like they're not there.
Love that M60 torque!!