Though it's been mentioned in the comments, I'd like to call additional attention to Stephen E. Braude's reply to Tom Butler's criticism of his work with Kai Mügge. His detailed response can be read here. (It has been posted via Dropbox, but you do not need to have a Dropbox account to read it or download it.)
In contemporary parapsychology, Braude is one of the very few serious researchers into physical and materialization mediumship, areas that have historically been plagued with fraud and ambiguity. For several decades the Society for Psychical Research largely refused to conduct any investigations into such claims, having found that virtually all cases they did examine proved fraudulent. The credibility of mediumship in general was severely undermined by public exposures of fake mediums of the physical/materialization type, carried out both by enterprising newspapermen and by Harry Houdini. The decline of widespread public interest in Spiritualism dates to this period.
Accordingly, it's only common sense to approach the subject with a degree of skepticism (healthy skepticism, not dogmatic Skepticism). And it's essential to be honest and open about any findings, even if they might be harmful to the medium or to some larger agenda. At the same time, owing to the tricksterish nature of these phenomena, it's necessary to avoid jumping to conclusions or painting with too broad a brush.
This is a difficult balancing act, but Dr. Braude has managed it throughout his career. His ongoing investigation of Kai, whatever its controversies, is one more important contribution to this perplexing field.