recently used one for a new Boiler vent ... cost 28 quid for the day . drill and diamond core bit..
had to go thro' a 18"" stone wall ... was hard work and took a long time ( arms were dropping off at the end) but did a good neat hole .
You do need a very powerful low speed drill to do this !
As said before ,take your time, there's no real clearances on the bit to get the dust out , make sure you stop before you hit the depth of the core bit , then break out the cut section with a chisel .
I found it best not to use anything as a lube as it tends to gum up the works .