Firstly, I apologize if opining on this thread is out of place for me. I do not have an E39 M5 but the closest cars I do have experience with is an E60 545i Sport and an E38 740i Sport. My E60 got totaled a while back when a 77 year old drunk driver ran into me. However, I have owned my E38 since it had under 30,000 miles and now it has 180,000 miles. I drive a total of 40,000 to 45,000 miles a year due to the nature of my business and for the past couple years the majority of them have been on the E38. I was just getting some pro-active maintenance taken care of at my indy mechanic here in the Seattle area- a preventative cooling system overhaul and I thought I would ask him how much I have spent there since I started going to his shop beginning January of 2009. The total was $17,145.47. Add to that 3 sets of Michelin Pilot Sport A/S Plus tires purchased every 30,000 miles or so within that period elsewhere and the total is upwards of $22,000. That is atleast twice the value of the car. I should say that my car is in absolute perfect condition- there are no leaks, no noises, no rattles, no squeaks, no rough shifting, it pulls like a freight train and tracks like one too (it ain't no M5 though). I absolutely love this car and keeping in mind the amount of city/highway/traffic miles I drive, it is the steptronic in this car and the amazing comfort/contour seats that keep me from getting an E39 M5. Even though my maintenance total is grander than a lot of your guys' , on a per mile basis I am sure I am spending less.
On another forum, someone asked whether I was overspending vs the value of the car but theres hardly any wisdom to thinking of it that way I think. If I start tempering my maintenance to keep it in line with the continuously decreasing value of the car, I would never enjoy my car as much as I do. I have zero interest in replacing my car with an E65 and an F01 is too expensive for me. In fact the only cars I would ever replace my E38 with are an E39 M5 (not sure about the manual transmission), a X5 4.8is (already have a LR3 in my garage- too many SUVs) or a Porsche 928 GTS. But until I do, I am almost certain, and after reading this thread, completely certain, that if you love the car, it really is money well spent. I can't even imagine driving a leaky, shimmy-ing, rattling car just to keep maintenance costs in line with the value of the car. I believe you will never be happy with a car in that shape, whether its an M5 or an old Chevy and will most certainly spend more money buying a new car you may or may not enjoy as much!
BTW, I love E39s- I was reading a review of the F10 550i somewhere and the author said that the F10 is just two generations too far from perfection- priceless! Similar things were said about the E38 but I will keep them to myself as this is an E39 forum.