jlee- I traded in my e39 2000 540iA. I loved that car. With Bimmers, it's all about preventative maintenance and good record keeping. My friends have e39 M5's and those are great as well. Mine was only modded for looks, lowered, rims, and stereo, but having the v8 was great. I don't have a heavy foot so I could get 18-19mpg per tank. My commute is a mixed of highway & city. A general rule my mechanic (formerly a mechanic at BMW Seattle) would tout is that he'd take a higher mileage BMW **WITH** records, over a lower mileage with **NO** records. That said, the 540/M5 being v8s are prone to valley pan and valve cover leaks. 540i's specifically have weak cooling systems so if it has over 60K, check to see if they've been replaced.
Blueduck- Right now mods are limited to:
1)tinting the front windows to match the rest of the car
2) Nokya Yellow fog light bulbs
3)USA Spec Ipod adapter
4)Momo shift knob
5)Thule Ride on Adapter for the roof rack (Bike trays)
Eventually down the road, WeatherTech mats, and possibly headrest monitors for the boys. I'm really just starting to get used to the car and enjoy it for what it is.