Never used one. I gauge tension by resistance of spoke key as i rotate and, after job done, as i use finger and thumb to go round all spokes to relive tension. sort of, playing the harp
So, my body/mind is the tension gauge.
You see, because no two spokes are 100% identical and, no rim is 100% round and perfectly formed (ok, this is microscopic stuff) then no two spokes are 100% of the same tension. It is the process of getting the rim balanced and true that really counts. And if the spokes are coincidently of uniform tension then, great.
For example, after running a wheel as true, ever noticed how some nipples have more spoke thread showing(at the top), or less? This is counter intuitive but, proves my point above.
After years of practice with stripping wheels, rebuilding wheels and, generally handling such products, you get adept at tuning your brain as to the rights and wrongs as each and every wheel will have been worked on by as many people, and then some.
However, this is based on 32 and 36 sopkes wheels that I am familiar with. A tension gauge may be quite essential in other applications. Or peace of mind.
Ride yorkshire and the north east
Visit the regional calendar here for details---> Update coming soon for 2015