The 5 series was a wonderful idea, but was over engineered, and thus destroyed any chance of the car being reliable or durable. The days of BMW's being cars that can last for generations ended starting with the E39 5. The car is beautiful in shape, and the interior is equally nice, but that is as far as it goes. It's pretty to look at, but you don't want one. They are now plastic, throw-away cars not intended to serve you longer than your lease agreement
The seats are made of very nice leather, but are so hard and provide no lumbar support, thus causing backaches and numbness in drives over 30 minutes long. In fact, the rear seats are much more comfortable than the front.
BMW tried to tap into the American market craze of "100,000 mile" maintenance intervals with the E39. They did this in ways like having a "sealed" transmission. There is no way to check the transmission fluid level, or any way to conveniently refill the fluid.
The M62(V8) and the M52(I6) engine cooling systems are also sealed, and thereby pressurized once the engine warms up. The cooling system is extremely prone to fail on several counts with absolutely no warning. The water pump impeller is made of plastic, and will shred over time, or if the impeller shaft bearing begins to fail. The pump shaft is mounted on a single bearing, that if it should fail, will crumble, sending the edge of the clutch fan into your radiator, thereby destroying it. The radiator itself is made of aluminum which is good, but the end tanks are made of plastic which fail repeatedly with no way to repair them, simply replace the whole radiator at a cost over $250 ea. The "expansion" tank for the radiator is also made of plastic, and over time the heat and buildup of pressure in the cooling system causes the expansion tank to fail, again with absolutely no notice and no way to repair it, simply replace it at $80ea.
Next item would be the intake manifold gaskets. Again the design is sound, but the execution is poor. BMW chose to use a style of gaskets that will dry and crack over time, causing many "rough idle" frustrations, and cost over $1,000 to have a dealer replace them. This would have been avoided if only BMW had modified their design to mimic what the rest of the world uses, O-rings!
Then we have the multitude of sensors that fail well before that "100,000 mile" mark of scheduled maintenance intervals. Many, if not most owners, have sensors fail within 30,000 miles. Many of which will leave you stranded on the road with no "Closed Loop" operation of the cars sensors so you can "limp" to a repair shop.
Now we come to my most favorite item to dislike about modern BMW 5ers, the front suspension. Bushings are plentiful and blow out in as little as 10,000 miles, causing an array of driving woes. Most frequent are the highway speed "Float" most commonly attributed to wheel bearings going out, and the infamous 40-50mph "Shimmy" in the front end that is usually cause by one of those super cheap bushings BMW uses.
There are many owners (Mostly young kids that have nothing better to spend their money on) that like to say "You have to pay to play." I do not subscribe to that theory since all of the troubles I noted above are NOT part of the regular maintenance items scheduled for service in your owners manual. These same "Pay to play" owners will also say that these items and the like are part of "Preventative Maintenance" when owning one of these cars. Again, I do not subscribe to this. What other car can you think of that recommends changing the radiator on a regular basis? Or completely rebuilding your suspension every 30-50k miles?
While there are many positive accolades that the BMW can claim, the rewards of a spirited drive in a BMW E39 5er do not out-weigh the frequent and often expensive downtime of owning one.
Before owning this E39, I have owned two other BMW's. An E34 525i and an E36 325i. Both served me well and gave little or no troubles. I sold both of them still running great at over 250,000 miles each. I just sold the E39 yesterday, and didn't even watch it drive away, I was just so happy to be rid of the millstone around my neck.