Just how effective is Rogaine/Minoxidil for hair loss? And how gradually/quickly will kept hair be lost if I cease treatment?

All drugs have side effects.  Topical Minoxidil (Rogaine) and Finasteride (Propecia) is not an exception.  Avoiding a drug due to it's "potential" side effect should be balanced with the "potential" benefits. Rogaine requires twice a day application.  If it is an inconvenience it may not be an option for you.  If you are worried about the side effects more than its benefits, it may not be an option for you.  These drugs only work as long as you use them. If you stop using them you will lose the hairs you may have gained.There is no cure for male pattern balding. If you know this, you can be a smarter consumer.
Medications such a Propecia or Rogaine can help and for some people it may delay the balding for many years.  The effectiveness of these medication vary widely depending on the individual.  

Dependence on minoxidil occurs for any new minoxidil induced hair growth. If you stop it after the dependence occurs with new hair appearing, the hairs that you grew will slowly fall off in the next few months. This will only impact the minoxidil induced hair growth, not your normal hair.