I have these as well. Going on about 10 years too but I have had to replace a bulb here and there. I bought an extra string when originally purchased them to use as spares. Have this thing as well, haven't used it on the outdoor lights but have on the indoor ones and does work to fix strings and identify bad bulbs that make the entire string go out.
Had these going on 10 years... never even had a bulb go out.