Well all things come to pass and I finally installed the USB/SD/Aux-in Xcarlink unit having received it in early December.
I'm happy to report that it works just fine. The installation is straightforward - simply plug it into the blue socket in the back of the head unit and, Bingo, the stereo recognises it as a CD Changer. I have placed it in the glove compartment which necessitated a bit of drilling into the roof of the glove compartment (this goes against the grain but needs must!).
The sound using either the USB or the SD is a bit lower than the radio and/or the regular CD but not enough to be an issue. With both music sources the steering wheel controls work well. For the Aux-in I tried my Zen MP3 player but found the sound a fair bit lower. Also, the touch-sensitive controls on the Zen would drive me nuts when on the move and would surely send me into the ditch!
I have hooked up my HTC Desire smartphone and this works very well. I am using an mp3 player app called PowerAMP and this has large buttons that are easy to operate when driving. PowerAMP also has a preamp and when the volume is set to max on this......Hey Presto......the volume is level with the radio and the regular CD. As a bonus, the HTC Desire fits tightly, when on it's side, into the open cigarette lighter area while the screen adjusts to a landscape display and it mutes the music if a call comes in. Couldn't be better. I have ordered a 32 GB memory card for my phone and reckon I will use that as my main source of music.
Hope this is of interest,