I also only write reviews after first asking my companion if she would like me to. I also do lots of research before each date and have never had a bad hobby experience so i never been in the position of having to write a bad review. Other peoples reviews have been very helpful to me in chosing a companion that I think I will connect with and feeling that connection is for me critical to having enjoyable sex. I write reviews to share my experience with other men and, if it helps, to bring attention to my companion so that others can seek her company. One woman wrote to tell me that after what I had written, her phone didn't stop ringing and to thank me.
What do I try to write? How do I rate a session? Difficult questions.
I still can't quite feel comfortable rating a woman or the intimacy that we have shared as I would a movie or a new car. The best I can do is to try to describe my feelings about  the time we have shared. Did she make me feel comfortable and at ease in her presence? Did she stir in me an emotional desire that translated into a stiffening of the right place and a physical wanting? Was she skilled in leading to my sexual fulfillment and an exciting finish. Did i sense her desire for sexual fulfillment as well? Was she genuine or a good actress?  I hope that I have been sufficiently astute and articulate enough to capture those thoughts in my reviews.
Ideally we should not need reviews. But from a practical point of view, they have been helpful to me and I hope that the reviews that I have written have been helpful to others, both the men who read my words as well as the women about whom I have written.  
A bit long but I hope this makes sense.