I have mentioned this in my Project thread, but I thought someone doing a search on this subject in the future might find this useful since I've had a few people already ask me various questions about the process.
My X5 is on 120k and is a 2004 facelift Diesel Auto.
Last years MOT (109k) stated that the Front N/S CV Boot was "Deteriorated" but just passed it on an advisory. This was before I owned the car.
Doing my pre-MOT inspection this year I noticed the N/S CV boot had not been replaced and had dumped all its grease over the inside of my alloy wheel. But not only had this not been changed, but the O/S CV Boot was also now split and had also dumped out all it's grease. I had also noticed a knocking noise when turning at slow speed.
Here is a picture of both the CV joints/boots removed from the car:
N/S Front Outer:


O/S Front Outer:


As you can see, both outer boots were completely gone and were definite MOT failures.
I had previously read that the Inner boots generally last a lot longer since they don't twist and turn like the outer ones.
This was indeed the case on mine. Both the Inner boots (even after 120k) were still in great shape and felt strong and flexible. (I assumed by now they would be stiff, leaky and probably on their last legs, but they look like they have years left in them).
This is how the Inner boots looked after 120k:

I originally was going to replace just the outer CV Boots but I decided that if my O/S Boot had been leaking for over a year (and was already knocking when turning at slow speed) the CV joint itself was probably fubar'd. It would have been a waste of money, time and energy to fit a shiny new boot/gaitor over an already knackered CV joint.
I then found a company on eBay (carspares-direct) selling 'OEM Quality' fully complete Driveshafts for £79.99 for the pair, with free Next Day delivery and 2 year Warranty!
http://www.ebay.co.uk/itm/2x-BMW-X5-E53 ... 3819wt_983
I asked my brother about fitting, and he said that in order to replace just the CV boots, you practically have to strip down as much stuff as you would if you were replacing the driveshafts anyway, so for a little bit extra work, you might as well get the whole driveshafts changed, especially at £79 for both. (I had heard BMW charging £300 a side to replace just the CV Boots)
So I bought the eBay driveshafts and they arrived the very next morning. My brother was going to fit them for free, but I ended up paying £80 at a local garage to get them fitted while they were doing my MOT. (I had previously had 2 other quotes of £100 and £80 at other garages to fit them, so around £100 seemed to be the going rate)
Here are the New driveshafts:



Now they are fitted I can confirm the slow turning knock has totally dissapeared, and it of course passed the MOT.
The garage that fitted them even commented on the great quality and could not believe they were £79!
Not bad for £159 in total (£79 for the driveshafts, £80 to fit)

Before I replaced the driveshafts, I was also a little concerned about the state of my CV joint after reading the post below from fellow member x555:

[quote=""x555""]I was driving off for the day yesterday with mrs x555 when 10 seconds in to the journey i felt terrible vibrations and a nasty shudder from the steering wheel accompanied by some awfull noises
.Fearing the worst i turned around and limped back to base ditched the car and then took off on our journey in style in the van


After an inspection on our return last night i couldn't see anything too obvious (was half expecting to see something like a wisbone or drop link hanging off the noise was that bad

Now I at least have peace of mind

Hope this thread is helpful to someone with the same problem anyway!
