I've tried one of these.

Yes, it plugs into the CD Changer connections.

With Analogue DSP it will work no problem.

With Digital DSP you would need a A->D converter and you would need some way to connect that up to the cable, probably easiest to cut off the existing connections from the Gateway harness and connect up to the A->D using RCA plugs. The OEM CD retention facility would NOT work without a D->A converter as well, so you might as well go for the Gateway 300 if you have Digital DSP, save £30 and lose the CD Changer.

Audio quality is good.

Build quality is so-so, the case comes apart very easily which is a bit of a worry.

Device switching is 'interesting'! Devices have a priority, iPod highest then USB, if neither are connected then the AUX source is selected. To switch to the OEM CD Changer there is a switch on the Aux input 'dongle', change the switch, turn off the ignition, turn it on again and you have the OEM CD Changer, to go back reset the switch and toggle the ignition again – impossible to do while driving

Head unit display is similar, if not identical to the ice->Link, so on the Nav that means font size changes when scrolling which is VERY irritating.

iPod/USB navigation is a bit tedious if you have any number of Artists/Albums/Play lists, but OK if you just have a USB memory stick or iPod Nano.

Personally I still prefer the Intravee (www.intravention.co.uk), although USB support is still not available due to continued delays with Alpine and the KCA-620m.