Most are--but some are not, such as the prophetic Mirror of Galadriel. The hobbits gaze into it to see the future. Sam sees the destruction of the Shire, and Frodo sees Sauron, but the mirror isn't seen as being an evil power, merely a neutral one.
On the one hand, since the mirror is being seen as a "neutral" thing, it could be considered as a "reality" of Tolkien's sub-creation; or even as a gift of prophecy. On the other hand, it smacks of divination. Sounds like a poorly chosen device to me. (Don't give any more away! ;-).
Also, I wish that Tolkien had given Gandalf a different title, other than "wizard," even though "guardian angel-like" is how I would have described Gandalf from reading the first 50 pages. It would certainly be misleading for children.