I have a 2006 330xi with 65K miles and got the engine overheating issue so I just replaced my Water Pump and Thermostat. Here in Canada BMW wants $800 for the Water Pump, $170 for the Thermostat and $30 for the screws!!! After dropping a grand on the parts I didn't want to pay for the labor and I enjoy the occasional mechanical project so I gave the DIY a shot.
The procedures here are good and I read most of the comments from folks that have done this.
My two cents from my experience are:
1) It's not hard but you have to be prepared to get dirty and have coolant spill all over and run down your arms.
2) remove the fan - it's easy to do and frees up a lot of extra room.
3) I found the power steering line mounting bracket a challenge to remove and replace since the position of the mounting bolt is really hard to get at.
4) the hoses that are attached by regular hose clamps are hard to get off - I found that by wiggling a very thin screwdriver under the hose that I could then pry it off. The hoses are flexible and can take a little abuse.
5) The two hose clamps on the water pump are the biggest challenge. You only need to remove one hose from the water pump to get it out since the other end of the second water pump hose goes to the thermostat.
6) When you put the parts back attach the water pump and thermostat connecting hose before you put it in. That leaves you with only one hard to get at hose on the water pump.
7) I found that reconnecting the hoses was a lot easier than taking them off if for no other reason than I could pick the most convenient orientation for the hose clamp bolts.
8) You don't have to remove the metal pan under the engine to do the job - I had to though as it seemed to be good at catching dropped bolts.
Thanks for everyone that shared their experience - It's what I like the most about the internet...