I've just assisted a school in replacing some legacy Smartboards and Projectors with new V4 Prometheans. School did have a couple of other Interactive screens recently installed (i won't name the manufacturer) but due to the changes with Smart software etc welcomed seeing the Promethean screens. Im pleased to say the school were blown away so much so that they purchased more screens than originally planned. I disagree with the above with regards to the external android. You can customise it at you will allows you to install the Promethean Store and Google play store so apps/games/ lesson content is easily added straight to the device. It allows you to mirror any kind of tablet Apple or Android directly on to the screen and move around the classroom at your leisure and in turn doing away with the need for a visualiser.
Another benefit of the Android being external is the fact you can upgrade it. Think of the life expectancy of the new screens etc. in two years that Android will look dated similar to your mobile. This way you can upgrade that whereas with an inbuilt Android your're going to be looking at a very dated OS in 5-8 years time.
Good thinking by Promethean and a great product. Best on the market at the minute. Equally though I'm looking forward to how Clevertouch, Smart etc will improve to bounce back.