Various versions of the quadrotor [pretty sure that AirRobot in Germany started the genre] are around although with development they now have three, six or eight rotors. They typically cost around £30,000 but the 'toy' versions can be had for much less.
They excel in searching buildings and places those little IED tanks cannot get on their tracks. So, great for the BD types.... why put a human or dog at risk when this will do the job for you?
They are extremely light so that any unintended impact with a human is unlikely to result in injury [although the newer ones lose this advantage - like everything they are getting bigger].
Operationally though they have a flaw when exposed to sunlight and wind... in that they are not allowed to leave the operator's sight and sound and anything over a breeze will blow them into the next county. A number have already been posted missing and once out of range they autoland.
So great fun kit in its place and cheap even for the professional versions. But like all flying machines very liable to interest the CAA when they see some licence fees in the offing. There are regulations about their use on the CAA web site. Long document and a number of police forces have come a cropper operating them outside the rules. Merseyside have been in the news about their errors.... they seem to have solved that though... the machine got away from the operator and neatly 'autolanded' into the depths of the River Mersey. Glug glug.
No wonder they have volunteered to give up their EC135T2 helicopter as well!