Here's my update for anyone interested in getting one of the xcarlink kits.
The install is really simple, plug and play, although you have to remove quite a bit of the console and the glove box to get to the back of the stereo, but it's all easy stuff and clips back in place easily. I managed to get a couple of scratches removing bits of the console, despite putting quite a bit of tape around the end of a screw driver, but I guess that's why there is a tool sold for the job.
It works pretty much as advertised, but there are a few annoying things in terms of functionality. The bluetooth connection is the same as for most of these sort of devices and once setup can be set to autoconnect in future. I can stream music directly from my phone via bluetooth, but doing so loses the steering/stereo control functionality, so I can't rewind, fast forward, etc. unless I do it on the phone and the display doesn't show track time/elapsed time. I can use the USB or SD Card slots to play music with the steering control/stereo control functionality and something meaningful on the display, but I can't use the bluetooth module at the same time, so I would be able to use the BT hands free phone function, whilst playing music. As far as I can tell, I can still use the USB port to charge the phone, whilst using the bluetooth connection, but I need to verify that. When streaming music via bluetooth, if a phone call is received I can use a button on the stereo to answer the call, which also pauses the music, but if I am listening to the radio when the call comes in, it doesn't automatically override, so I have to switch mode to the external input and then press to answer the call, but I guess it doesn't take too long.
Despite the few niggles with functionality, it does do what I want better than any other product that I have come across, with the exception of a replacement head unit. Fortunately, I have an HTC Touch HD pda phone, which has a large screen and comes with Windows Mobile Media Player, so I have made a skin for media player with huge buttons and I've ordered a magnetic dash mount, so that I can use the phone display to control music, as easily as if I was using the stereo to control it.