My Nov 2004 E60 has 120,000 on the clock now, I've had it for just over 3.5 years now - firstly regarding build quality, there is not a rattle out of mine, the car is as tight as it's ever been. No peeling, corrosion, squeaking etc. Each one is obviously different judging by what I'm reading above. The only thing is that one of my cup holders won't open - can't remember if it's the left or right one since I never use them anyway. But that wasn't working when I bought the car at 65k miles.
Electrically, I've needed a new slip ring in the steering column and a new wiring loom in the boot lid after the old one corroded. I consider that lucky given the age and complexity of the thing.
Mechanically it's been grand for the past 20k miles approx and has really settled down, but up until then the car was a disaster.
Turbo replaced twice (first one blew, second one started squealing when cold a year later which was replaced under 2-yr warranty on dealer-fitted parts).
Inlet manifold replaced twice (second time after me complaining to BMW themselves about dealer incompetencies, had a couple of other things changed as part of a goodwill gesture).
Exhaust manifold.
Sump gasket.
I've had a few bits like control arms and track rod ends replaced around 110k there. I asked my mechanic to replace 4 glow plugs (they were all faulty apparently) and clean the EGR valve at that time too, which didn't seem to make any difference to the car's startup or smooth it out any more but at least I know it's done. Injectors are still fine.
I think it's a great car, I love it to bits and in a way don't mind putting money into it since nobody else is going to be getting the benefit of owning it after me. I like the interior. In the back it's a nice place to be, in the front it's fine, I'm just that used to it I guess. The sports steering wheel greatly helps though because as Fox says the SE one is dire, the extended interior lighting is nice but not a necessity. On the outside, mine in Sport guise is still a very fresh looking car, the SE (particularly pre-LCI) is definitely getting on now.