By the time mine hits 150K I will have replaced almost every part LOL. Not really, just seems like it sometimes. I'm almost at the 100k.

I just it 100k last week (04 manual transmission), its an achievement for me. So far I've only replaced the water pump, I did have an incident last year where my oil spewed out of my oil cap due to condensation (very scary in the middle of winter lots of smoke). I am a bit worried about my transmission fluid though since I hear a strange "woop woop woop" sound when I am at speed, and some folks said it could be the fluid.

Nice post, and the PMs of the cooling systems are critical! I would recommend to everyone to replace water pump, thermostat and hoses at 100k. Mine went out on this same car this week, and now am looking at a replacement engine.
Good luck finding a newer x5 with the same reliability. It is luck of the draw from all I've read.