I noticed something interesting recently where after a person I hired to do a job completed the task he was asking for me to write a positive review for him on various sites. What this made me think of was all the times where people write a positive review for a product or company only to then come back say a month later saying how they want to retract their statement. With that in mind, I think when it comes to online reviews and endorsements like these that you should sit on it for awhile to see if it truly ends up delivering in the long run.
I think this kind of gives people the incentive too to actually make sure what they are offering you is truly good and not simply making it look good for the moment as they want your money, so to speak. In some ways I think you should hold off too because your endorsement probably means a lot for another person to try the product/service as well. So there is a little bit of responsibility on your end if the company is giving people lemons.