Here is my experience with the Jehovah’s Witnesses.
I first met a Jehovah’s Witness when I was in college, and the nice lady at the door said to me, “I was once just like you; I’ll come back next week, and we will have a weekly Bible study.” And that is exactly what happened.
However, after several weeks, the result was an impasse. We simply disagreed on the interpretation of numerous Bible verses, but by and large, they were good conversations.
The only time that I recall that things became contentious was when I pulled out some of the books that I was studying (some of which were written by former members), and I was immediately advised that it contained lies.  While I can understand that viewpoint, it was nevertheless my duty to hear both sides.
I can understand that members would be restricted from such books, but I was not a member, and I felt a sense of duty to hear both sides, before becoming a member.  However, one time when I discussed my research, I was actually yelled at, and which was quite surprising to me. What happened was that I mentioned that I read where the author of the Commentary on the Letter of James had been disfellowshiped. That was an ugly moment. For me, it was just what I read, but for the other person, it seemed like it was taken very personally. But here is what I read:
David A. Reed: “Commetary on the Letter of James was written by Ed Dunlap, according to former Governing Body Member Raymond Franz in his book Crisis of Conscience (Commentary Press, 1983, p.228). Dunlap had served prominently for decades as an instructor at the highest levels of the Watchtower organization, responsible for teaching missionaries and men who run the Society’s foreign branch offices. But his frank honesty and his adherence to the Bible rather than organizational tradition led to his being convicted of apostasy by a Watchtower judicial committee in 1980, and to his expulsion from the sect a year after his book was released (Crisis of Conscience, pp.236-289).” (Jehovah’s Witnesses Literature, Baker Books, 1993, pp.159-160, emphasis mine)
Raymond Franz: “So they disfellowshiped Ed Dunlap, and he was asked to leave what had been his home at the Bethel headquarters. He returned to Oklahoma City where he had grown up and where, now 72 years of age, he supported himself and his wife by hanging wallpaper, a trade he had practiced before he began his forty years of service as a full-time representative of the Watch Tower Bible and Tract Society.” (Crisis of Conscience, 2nd Edition, Commentary Press, 1992, pp.287-288, emphasis mine)
So I know that I wasn’t lying, but only relaying what I read, in my research, of hearing both sides.
So the impasse was primarly in regards to a disagreement over the interpretation of various Bible verses but the issue of date-setting and false predictions played a large factor as well. One of the key teachings of the Watchtower is the alleged invisible return of Christ, which allegedly had taken place in 1914. Upon further research, I learned that this date was originally predicted to be the date for Armaggedon, and then later reinterpreted to mean an invisible return of Christ, and then the generation alive in 1914, would not all pass away before Armageddon eventually takes place. Here is some of that research:
Reasoning From the Scriptures: “They believe that we are living now, since 1914, in the last days of the wicked system of things; that some who saw the events of 1914 will also see the complete destruction of the present wicked world; that lovers of righteousness will survive into a cleansed earth.” (Watchtower Bible and Tract Society of Pennsylvania, 1985, p.200, Section 8)
Reasoning From the Scriptures: “If Someone Says---‘My minister said that Jehovah’s Witnesses are the false prophets’ You might reply: ‘May I ask, Did he show you anything in the Bible that describes what we believe or do and that says people of that sort would be false prophets? ... May I show you how the Bible describes false prophets? (Then use one or more of the points outlined on pages 132-136.)’ Or you could say: ‘I’m sure you’ll agree that specific evidence should back up such a serious charge. Did your minister mention any specific examples? (If a householder refers to some claimed ‘predictions’ that did not come to pass, use material on page 134, and from the bottom of page 135 to the top of 137.)’ Another possibility: ‘I’m sure that if someone accused you of something similar, you would welcome the opportunity at least to explain your position or point of view, wouldn’t you? ... So may I show you from the Bible...?’” (Watchtower Bible and Tract Society of Pennsylvania, 1985, p.200, Section 8)
For me, based upon the visible predications for the end of the world in 1914 and 1975 not coming true, made the credibility of a predication for an invisible and unverifiable return in 1914, doubtful. And this is besides the point that Revelation 1:7 indicates the Christ’s return will be anything but invisible: “Behold, He is coming with the clouds, and every eye will see Him, even those who pierced Him; and all the tribes of the earth will mourn over Him.” Those who pierced Him, will see Christ’s return from their place of judgment in Hell.
However, in fairness, the reverse could also be asked: If Jesus was not, then how would that change your life? The answer from a Christian is that it would change everything, since a Christian’s life revolves around Jesus, that is, believing in Jesus, praying to Jesus, asking Jesus for forgiveness, ect. The Bible tells us to “believe” in Jesus (John 3:16) and to “know” Jesus for eternal life (John 17:3), and if Jesus was not “God” having become flesh (John 1:1, 14) and the “fullness of Deity...in bodily form” (Colossians 2:9), then it would drastically change a Christian’s life, and how one identifies with God.
But here is the bottom line:
“So Jesus answered them and said, ‘My teaching is not Mine, but His who sent Me. If anyone is willing to do His will, he will know of the teaching, whether it is of God or whether I speak from Myself.’”
It is for this reason that no one will have any excuse.
 I was meeting every Saturday with a Jehovah’s Witness, who was very nice and intelligent, and we good conversations, but one week when he brought his Dad along, things turned a bit sour when I had mentioned one of the books that I was studying, which discussed key Bible verses pertaining to our discussions, such as Colossians 1:15. That book was Jehovah’s Witnesses Answered Verse by Verse.
 Here are examples of critical materials that I find to be very helpful.