I suppose the main downside is that it doesn't do what iDrive does, it would have been a great product if it replicated exactly what the OEM system does after all it does have a canbus input.
I use mine every day, but to be fair I use it with Android Auto so I'm not actually using the screen to it's full extent. Sound quality is very good, although I'm sure a few audiophiles would say it's not up to much, but to be fair when your driving along with the roof down what on Earth are you going to expect from it?? Seriously though the sound from it is very good but is limited only by how good your headunit system is in the first place.
Would I buy it if I already had iDrive?....... NO, there are other add-ons to get what I'd want out of it.
Would I buy it if I didn't have iDrive already?...... Ermmm I'd probably say yes simply because it does bring a lot of functionality that you simply don't have. Yes you can stick a tablet in the car but it would be a bit of a hash to sort it and this screen does fit really well and it does look OEM as opposed to something you've thrown together with some toilet rolls and sticky-backed-plastic!
It is costly, and to be honest I think it should be a lot less considering it's not exactly new technology (the actual tech side of it has been around for a number of years now).