I had been doing chambering for more then 30 years without one, but a year or so ago I saw a product review at the gunsmith forum The Gunsmiths :: Index
Most of the members are professional gunsmiths and I find they know what they speak of, so I bought one and it was well worth the money. First thing I liked was the workmanship of the tool, second was the accuracy and the ease of use. If you want a accurate chamber I highly recommend them. I know that the TAOGART gunsmith apprenticeship program www.taogart.org
lists them on their recommended tool list as well. And no I have no financial interest in the tool just think when you find a good tool or fixture with the quality of this one the word should be spread.