It can be. In lap land - it is a common practice to repeatedly move from a sauna into a cold dip in a river. This process can be used as part of a detoxification program. Changing back and forth from a cold to hot shower is a similar detoxification protocol.
Not necessarily. It is definitely unpleasant especially for a novice to this practice. I am not aware of any positive effects of cold showers except for possible training to endure cold temperatures in someone who may expect to be exposed to those w/out having adequate clothes. Some popular beliefs are that cold exposure may raise one's body resistance to colds, but that's more of a unsubstantiated myth.