SYMPTOMS:
Just last week, my girlfriend is driving along, about a half hour from home and a yellow warning appears on the dash. "Engine Temp High - Drive Moderately" or something like that and it goes into limp mode. Soon after a red warning appears, "Engine Overheat - Stop". Yay...
64,500 miles
Luckily temps are low enough, a quick cool down and I tell her to take it home and put it in the garage until I figure it out.
The Internet! Not long after I search the symptoms does it become clear,
the water pump has failed.
-Engine oil temp on dash was perfect.
-No codes were thrown, no CEL.
So, without as much as looking under the car, I order parts and search for a DIY guide. (Dealer wanted over $700 labor)
I found several DIY guides for the 335i cars that looked simple enough. Move the sway bar out of the way and voila, swap it.
"Hey, I can do that."
PARTS:
1. Genuine BMW Coolant - 1 gallon $21.37
2. Genuine BMW Thermostat $93.39
3. Vemo Water Pump w/hardware $393.70
note: I went with the Vemo branded pump because it is literally the same casting and part number as the BMW one. I saved some money. All of these prices are ECS prices.
Total parts cost: $508.46
THE JOB:
Let me tell you now, folks. I got under the car, removed the undertray, then slowly cursed to myself. I could not see a clear way to get the water pump and thermostat out without moving the radiator. On the DIY's I found, the sway bar mounted from the bottom. Easy to get out of the way. Our X3 sway is mounted from the top, with little to no access from the underside.
.....
So...I put the undertray back on and dropped the pig off at a local Indy.
Defeat.
LABOR:
Replace Water Pump and Thermostat $415.80
Hazardous Materials $16.63
Total labor cost: $432.43
TOTAL JOB COST $940.89
I am a bit sad/disappointed that it cost this much and happened this soon for a simple water pump replacement, but that is the joy of electric water pumps that are hard to get to. I heard a rumor that the new turbo sixes are getting belt driven pumps, I hope it's true just so they 1. last longer and 2. are cheaper to replace.
All of you shoppers out there looking at the N55 powered cars.
You have been warned.
In the end, just happy to have it back. It really is a fantastic SUV that shames most others in comfort, looks, power, tech, utility, etc.
I am just happy this didn't happen on our recent trip to NYC and back! (2,000 miles) 
That would have sucked.