Jan_Tekin and I did this on our cars at the weekend - thanks to all who contributed, between this guide and the BMW instructions it was all pretty straightforward if just a little fiddly at times
Jan's is a manual, mine is an auto - no difference in installation. Oh, and make sure you ask Sopers to print and send the BMW instructions with the kit, or do a search on here as I think they are posted somewhere.
Couple of other things (may have been covered, may not...) :
1. It is much easier to get to the loom and cable ties behind the light control unit if you take the air vent out too. The loom is cable tied to the dash so doesn't have much slack - taking the air vent out helps get at the block on the back of the light unit to detach it.
2. To detach the block, use a wee screw driver to press down the catch, then get your fingers in and pull the white clip forward and down. Needs quite a bit of force, you'll know what I mean when you do it.
3. There's a lot of talk on here about what pin to use in the engine side connection. This may be because the BMW instructions are ambiguous. There are directions for the M54/N52 built up to 09/06, and others for N52 built after 09/06. Both route the blue cable to different pins. My car was built 02/06 and Jan's is an 07 - BOTH our cars used pin 39. Some of the electrical box internals on our cars are different, but the cruise control works in both. Perhaps a typo in the BMW instructions, as the M54 was not built in 2006 as far as I know...
4. Finally, the instructions say you need to use the 'repair kit' to extend the blue cruise control loom wire. We could not work out why as the connecters on the loom and the repair section were identical so we did not do this, plugged it straight in and works fine. You may well have to use the repair wire for the M54 engine, but I can't confirm this.
Hope this helps