PostgreSQL Performance vs. Microsoft SQL Server

A recent poster to the pgsql-performance mailing list enquired as to the relative performance of Microsoft SQL Server vs. PostgreSQL.  It's a reasonable question.  Switching databases can be a major project, and you certainly wouldn't want to do it and then find out at the end that you'd taken a huge performance hit and had to throw all your work away and switch back.  The good news is that this scenario is fairly unlikely.