I have been using them on and off for years and like them. Like mentioned though, I still use A good volt meter with A load and check my batteries after each flight.
Another good unit is the GEM 2000, I think it's still made? I have A couple still in my shop so I haven't had to buy one in years. I use those in my bigger planes because they can tell you of other on board problems. The price is A lot more then the VW so I don't use them on everything.
Someone may post up and tell us this answer. When I started using VWs they only had them for 4.8. {I think} And now today they have them in both 4.8 and 6v. Because I had A bunch of them I haven't had to buy any in years. I think I saw this in A magazine or the Tower book.
My question is are they either or?? 4.8 or 6v and do you have to buy them for the voltage you are using??
Guess I could look it up but I'm sure I'm not the only one with that question.