I am not so sure about the details on how this word originated. Of course it is the a loanword from Japanese, and it is a substitution of 中, but I am not so sure which exact word that including 中. In 萌百 it says from the word 中坊(Japanese) that means middle school, which is the same as Tang's answer because the 中 in 中二 means middle school.
But the usage is very clear. X厨 means insane fan(s) of X. The focus is the "insane" part. In the beginning it is a negative word. Some one accuse others to be too crazy about an idol/work. But later on it is used by the fans themselves indicating that they enjoy the idol/work so much and would like to be crazy.
For now, it is tend to be a neutral word, and just means "fan" and less stress on the "insane" part, but be careful that this usage is only when speaking of oneself. If you say "I am a X厨." It can mean merely "I am a (normal) fan of X", or "I sickly love X." (as Kevin's answer said) But if you say "This guy is a X厨." it is somewhat offensive.