Andrew don't do it.
I had one and the fit and quality were ok but not OEM. The grilles supplied were not all that either.
I used an OEM bumper and just cut it about a bit to get it to fit. Was'nt difficult to D.I.Y but any body shop would make easy work of it and the fit is guaranteed. You can change the main carrier like Darren did but its not essential but it is the 'proper' way (makes access to towing eye easier).
I was going for the facelift GT look anyway so the grilles were useless to me. So if thats what you want to do then its a false economy as youll have to buy the OEM GT grilles anyway and you'll get a secondhand OE GTI bumper for much less than £165!!!! I picked mine up on the bay with the splitter for £40, whatever you get its got to be sprayed up anyway? The OE grilles will fit the x-parts bumper no problem.
The existing fogs will fit with the x-parts grilles but not with the OE GT or GTI ones. I had to get new fogs and the connectors were the same as I had originally but I have heard some people had problems, so I think its VW changing mid run so its suck it and see on that one.
Someone only asked the other day, have a read through this
http://www.mk5golfgti.co.uk/forum/index.php/topic,22828.0.html