Hi Steve
Heres a few comments on your questions and from my own experiences trying to "integrate" current communications with a dated PCM2.0 system in my car.
1) John Kleis have been around for 25+ years that I know of so they must be doing something right, plus Northway would only recommend local companies that do a decent job, their reputation is at stake too.
2) I looked at various options to connect my iPhone to the PCM for music and hands free and two options seemed popular, Mobridge and Dension, the first was ruled out on cost circa £700, so I went for a Dension Gateway 500S BT it seemed to have all the functionality, iPhone / USB, bluetooth handsfree. Their delivery and technical support was great.
3) Installation, having looked on forums like Planet9 I found lots of guides on the audio interface but nothing on Bluetooth equipped units, I started out installing the unit alongside the CD / Nav units under the bonnet, audio worked fine and could control via PCM, but the Bluetooth signal wasn't strong enough to maintain a reliable connection (great outside the car !), so the unit needs to go inside. Then you have problems with the MOST optical loop, the unit has to be in a certain position to function correctly, and you need to loop out the existing phone module (need to remove drivers seat) to do this to eliminate error messages on the PCM. I never got as far as installing the iPhone connection box in the centre console, not a big job but lots of interior trim has to come out.
In short, leave it to a professional, and leave the car with them where they have the tools and space to carefully remove and store all the trim rather than in the rain outside.
4) User Experience, I have the unit installed under the bonnet again with an iPod connected works well for music but no hands free so I failed there, using a headset instead. The Dension interface is pretty poor takes ages to select artists or albums as everything is alphabetically driven, to be fair this is as much to do with the Apple software as anything else, I had the equivalent Audi system installed in a previous car and it was only marginally better, however if you can get the unit inside the car then you can use the iPhone menus instead.
5) Summary, would I buy it again, not at £300, it needs to be professionally installed and the total cost would exceed replacing the PCM with an aftermarket unit that probably has better audio and certainly a better satnav. However I wanted to keep the car as original as possible and aftermarket didn't look or feel right. It's two years since I bought the Dension maybe they have new products that solve the handsfree issue but I doubt the user interface will be much different. Not having the phone is a frustration but less of an issue than no music
Hope you find something that works for you
Mike