Not for the criminal activities, but for huge volume of transactions people started using it as banks were demanding for high transaction fee as well taxation issues.
but if you are avoiding tax or trying not to pay tax this is also an illegal activity, because states run on taxes and if you are not paying your legal tax that is not good for you for the state and for other people, but i will like to add that we are not paying tax on bitcoin because most of the states have still not policy about bitcoin or to impose tax on bitcoin may be the reason that still they have not legalize it.