I was an annihilationist at one point, and have since rejected it.
Why?
I realize now that annihilationists confuse types in the Old Testament with the reality.
What do I mean by that?
The OT often portrayed the fate of the wicked to be reduction to ashes through the fire of judgment. The wicked were no longer able to effect their evil in the created realm. Additionally, the imagery of being eaten by maggots was used.
However, the OT communicates in shadows and types. These are "fuzzy pictures" of the reality.
If the fate of the wicked is presented in such terms in the OT, and the NT provides more detail, these NT accounts are not contradictory. The OT did not communicate in a clearly focused manner on some topics. Eternal punishment was one of them.
So, if the NT indicates that eternal torment is the fate of the wicked, and that the wicked continue to exist in a state where they are shut off from the presence of God, we need not dismiss these Scriptures, and revert back to the OT shadows and types.
I would agree that if one views Scripture in a simplistic, flattened manner, annihilationism might seem reasonable. But when one understands the way the Bible is put together, and the employment of shadows and types, annihilationism doesn't seem reasonable.
For instance, I see a typology between the Garden of Eden, the Israelite camp, Jerusalem, and the New Heavens and New Earth. In each case, a certain space is defined as holy space. The unholy are placed outside of it. They are spiritually dead in the sense that they are no longer in God's presence, but they are conscious.
For instance, Adam and Eve were placed outside of the Garden of Eden after their rebellion. Unclean individuals were placed outside of the Israelite camp, as well as the city of Jerusalem. And, finally, the unrighteous are placed outside of the New Heavens and New Earth.
They are no longer able to effect evil in the created realm anymore. However, they still exist outside of the New Heavens and New Earth.
I suggest reading Revelation 21-22 in this regard.
Anyways, I believe the fundamental problem with annihilationists is their hermeneutic. They are viewing the type (physical destruction and annihilationism) as being the reality. They are reading the OT forward into the NT, instead of using the lense of the NT to evaluate the OT.
Additionally, it is plain to see that words like death, destruction, and perishing can be viewed in ways other than the annihilationist is viewing them. For example, Ephesians 2 describes the state of the man in Adam, who is not born-again. The first verses of the chapter describe him as being dead, because he is spiritually separated from God, and not fulfilling his intended purpose as a result.
I can understand why annihilationists believe their position, but I disagree with it based primarily on the above. Understanding typology within Scripture and how the NT uses the OT is important in regards to evaluating truth-claims like this.
By the way, I don't consider annihilationists to be heretical by definition, although many annihilationists are heretical because of their other doctrines. For instance, several major cults are annihilationists.