"what quality do you think a mentor should have and what are your best experiences and worst experiences with the program."

Alan's asleep so I'm going to have to wade through this one myself, but you might get better answers from him tomorrow (He's all about the Mentor program!). Qualities... well, obviously the ability to write good reviews yourself, which you already do. It also helps to know your way around the site really well, and to know something about the various clubs and programs available, because some students will ask you about them.

You also need the ability to keep things in perspective and let things go from time to time. Like, when you get a student who won't put sizes in reviews like you mentioned, no matter how many times you suggest that they do, you have to let that go. Your goal is to help them write better reviews than they did when they came to you, rather than to mold their writing into something that doesn't work for them. They each have their own style, and it's best just to let that flow whenever you can.

Last but not least, you really REALLY have to just want to help people. The Mentor program isn't like the Proofreading program. You don't get any points for editing your students' reviews, no matter how many times you do it, and you rarely see that $25 gift card at graduation since so many are either Advanced Reviewers already, or become one during class without you knowing it. Make sure you can be happy just knowing that you helped somebody.

For your other question, "best and worst experiences", the single "best" for me was the student who had dyslexia and told us that the reading/writing methods we taught him (which Al & I picked up when we were mentoring homeschool students years ago) helped him in other areas of his life. You can't imagine how good that felt. Other than that, the best is any time I read an X-student's review and feel that it's "Extremely Useful."
The worst was the student who signed up and then canceled after 2 days because Admin hadn't sent him an assignment yet and he got tired of waiting. He came back a second time, and wound up canceling AGAIN because he said it took us too long to edit his first review, which was also 2 days, out of the time Alan was in the hospital, which the student knew. Ridiculously impatient, that one.
Other than that, it's a tie between the students who disappear mid-class, or the one or two who went on to become unpleasant additions to the community forums.