Well I have not read any of the other replies so excuse if I repeat anything. I have mixed feelings on paid critiques but in the main I would not use this service. There are plenty scams who operate on a pay for critique service. $30 for listening to a song and writing a letter is nice work if you can get it. There are reputable people who offer a critique service but you are paying for a single opinion. That opinion will not get it sold and you had better make sure of their credentials and that they are aux fait with the style and genre of the piece they are presented with.
If a songwriter knows the craft and what is required of a song then there is little reason to have a critique in the first place. That is just copnfirming what he already knows.

It is important to test drive the song but that is different than a paid critique. To test drive a song I would use several trusted objective constructive opinions and take an average.
You know these people and can see where these opinions agree and differ.

http://www.soundclick.com/bands/songInfo.cfm?bandID=687727&songID=5238055