There is much to say about Jehovah Witnesses. They are recognized by all other major Christian groups as a cult and as unorthodox. They have a very subtle and deceptive way of arguing to convince you of their position.
It's best not to tangle with them theologically unless you really know your stuff. They are highly trained in the art of persuading people to accept their view.
If you really want to engage them, be sure you are very firm in your faith and know your stuff. Catholic.com has some tracts on Jehovah's Witnesses that may prove useful. They also have some good books.