I know nothing about the commercial ones--many of them are very expensive (considering the small amount of actual electronic components they may or may not actually contain). In fact, some of the commercial units are prohibitively expensive.
Having said that, I have had the experience of discussing the use of rods made from copper-coated steel/steel. During several fascinating discussions, I had the occasion to discuss their usage, for saving lives, with some Vietnam vets, soldiers on active duty that used them successfully to locate enemy tunnels and land-mines during the war.
Now, these items were made from rods that would cost a person next to nothing for the materials used (they were often made them from whatever could be found--heavy gauge wire, acetylene welding rod, etc.). The people that used them were very good at using them.
It seems that some people can find items with the rods, and some people just can't. However, the ones that can are often very accurate and efficient with their usage.
There are people that also use these simplistic methods to look for gold deposits as well. There's ample proof that the Spaniards used "dipping needles" to look for gold and silver deposits for hundreds of years--it (the use of rods and needles) many have something to do with electrical charges that build up around mineral deposits.
As far as spending thousands of dollars to buy a commercial set/unit goes, I'd be very hesitant to recommend that course of action. I'd recommend making your own set (for next to nothing) to see if you can locate items with them before you take any further action. Better yet, if you can find someone that's already good at using them, see if they can train you, or see if you can talk them into helping you locate items with them.
There should be information on the usage of rods available on the Internet. I figure that if people can find tunnels and land-mines with them (people also use them to find gas, water lines, and well-water), why not be able to find minerals with them as well?
I'm sure that others know far more about their use than I do, all the best,
Lanny