PITR performance overhead?

I am looking for some general guidelines on what is the performance
overhead of enabling point-in-time recovery (archive_command config) on
an 8.1 database. Obviously it will depend on a multitude of factors, but
some broad-brush statements and/or anecdotal evidence will suffice.
Should one worry about its performance implications? Also, what can one
do to mitigate it? 

Thanks,

George