Constant boosting is not a problem.  Having it ping is more of a problem.  That can cause damage and is the more likely cause of damage.  The booster can stain, but that won't hurt it.  Deposits will cause problems.  The easiest way to tell is to look a the spark plugs.  They are a good indicator of what is going on in the cylinder.

Over boosting can cause a driveability problems.  It can create hesitation and cold driveability problems.

As with any type of products, some work well and some don't.  Some boosters really don't boost much, and/or may cause a lot of deposits.  Again, look at the plugs.