A lot apparently.
Let's have a pop quiz:
When should you quote lovely glowing comments received from other agents in your query letter?
B: Sometimes: if they're personal
C: Sometimes: if they're spot on
Here, read a very calm, very cordial post on the subject by Jessica over at BookEndsLLC.
Want to take the test again?
When should you quote comments received from an agent in your query?
A: When you really want to connect with an agent
B: When you want to demonstrate other people have read and liked your work
C. When the agent said it was great, but didn't fit her list
D: When it clearly wasn't a form rejection, but a personal rejection
E: when you know the other agent you're querying is a friend of the one who made the comments
F: none of the above