You get to experience the new UI, whilst it's a better UI than any previous Android iteration, it's not one of 4.0's awesome features. If you could experience Android Beam, Panoramic camera and face detection unlock then I would agree.
Camera, WiFi, SDCard and Radio do not work - hardly seems like a good experience to me.
As per every Android version, there's two hardware minimums, one being the phone is based on ARM architecture and the device's ROM is large enough in size. As Google have already said, any device running Gingerbread (which does officially include the Desire seeing as HTC released an RUU) should be able to run ICS.
This is something you would flash to your phone and play with for 30 minutes before flashing back unless your Desire isn't your daily phone because it simply isn't good enough to run as a daily ROM.