I have them in .223, .243, .308, .30-06 and 8x57. They are very handy, especially when you have several rifles in the same caliber. Not all rifles have the same headspace. The Precision Mic will allow you to measure the headspace of each rifle and set up a sizing die accordingly.
As an example, I own about a dozen 30-06 rifles. Some, like the M1 Garands, have headspace at the SAAMI maximum, so I set up a sizing die just for those rifles. Others are about midway between SAAMI minimum and maximum, so I set up a separate sizing die for those rifles. I also have several rifles that are at the SAAMI minimum headspace, so I've got a separate die set up for those rifles.
If I shoot the ammunition that's loaded for the short headspaced rifles in any of the Garands, the brass will separate almost every time. Conversely, I can't chamber the ammunition that's loaded for the Garands in the short headspaced rifles, since it's .008" too long for those chambers.
The Precision Mic eliminates a lot of guesswork and solved some problems. I like them.
Hope this helps.
Fred