A few things to remember when commenting and rating the music libraries. I doubt any of them would guarantee you placement of your music, so I don’t believe not having your music placed is a reason to give a company an unfavorable report or low rating. There are many things to consider (in no particular order):
Do they communicate regularly?
Are they responsive to your queries in a reasonable amount of time?
Do they make you aware of opportunities?
Are you comfortable with their contract?
How easy is it to submit material – CD/Upload?
Are they effective?
Please feel free to add more.