I think a lot depends upon what you would mean by 'effective'. Did we have a multitude of people come to our church as a result of door-to-door witnessing? No. Probably in the years of doing it, I saw one to three eventually join the church.
But, we did share the gospel, 'planted the seed' if you will, with countless folks who had stated they had never heard it before. In a so-called "Christian" nation like America, I found that a lot of folks sort of consider themselves as Christian without actually knowing what it means to be a Christian. We would ask the folks the question, "If you were to die tonight, would God let you into heaven?" Most folks say yes right away. When asked why He would, the answers tended to be because most folks thought that they were 'good' people. Only most folks seem to define 'good' as not being an axe murderer. It's interesting to talk with people as to why they think they are 'good'. I've talked with folks who have been actively involved in just about every sin imaginable, with the exception of murder, and they define themselves as 'good', pretty much because there were 'worse' sins they weren't involved in. I specifically remember talking with one woman who had three kids, all from different fathers, none of whom were in the kids lives, was supporting the kids via welfare, prostitution and pushing drugs, was taking drugs herself and yet still considered herself good enough to go to heaven.
Talking with folks like her, we would explain that no one, not one person here on earth is actually 'good' enough to go to heaven as heaven is a holy place and one must be holy to be there. While the vast majority of folks call themselves good, I never met any who would describe themselves as holy.
This conversation regarding good versus holy became then the springboard of talking about what Christ did for us on the cross and how, by faith, we can be renewed and regenerated in a life that can bring us holiness. It was the first time many of the folks we talked to had ever heard the gospel explained to them and understand what Christ did for them.
Did many 'convert' right then? No, not many, although we certainly saw more than a few pray a prayer of repentence and acceptance of Christ. But, whether we saw the regenration of the person or not, they had the good news shared with them, and the Holy Spirit could then continue to work within that person.
And, I think that is the point of witnessing. Not to 'convert' folks, but to explain the gospel in a clear concise way so that folks understand Who Christ is, and what He accomplished and how one can live a life pleasing to God. It's always the work of the Spirit to convert and regenerate folks.
