According to the fatwa of the late Sayyed al Khoei, the grand marji and teacher of most current maraji of taqleed including Sayyed Sistani, when he was asked about the shaikhiyah and whether or not we are allowed to pray behind them his reply was: it’s not permissible. This should give you enough idea about them. They are very extremist in their views about the Imams and their level of knowledge about the unseen.
The name of the book is : Munyatu Al Sa’il