Critiquing or commenting on someone's talk Assume I attended someone's seminar talk which I liked a lot. Also I heard that yet another expert X also said that the talk is interesting. 
Is it ok to say to the lecturer that the expert X and I liked the talk, or should I not mention another's opinion?
Now assume the opposite situation: the expert X and I did NOT like the talk.
Is it ok to say to the lecturer that X and I did not like the talk, or again should I not refer to other people?
[update] Now assume that the lecturer explicitly asked me if I heard what X said about his/her talk. Can I tell the lecturer X's opinion (assuming X did not ask me in advance not to do that)?
X, being an expert in the field, can presumably find ways to communicate their opinions to the speaker, publicly or privately, if they wish to do so. 

Often, opinions communicated between experts will be much more complicated and mixed than a simple like/dislike. "I agree with you on points A, B, and C, but I think point D is contradicted by Y's research. I'll send you a reference." is much more likely than either extreme.

If you just heard X talking about point D and Y's research, you could get the impression X disliked the talk. If you just heard X talking up points A, B, and C you could get the impression X liked the talk. Neither impression would be fully accurate.

Unless X has positively authorized you to speak on their behalf, you should not do so. If asked directly, indicate that you are unsure and, as suggested in a comment, encourage the speaker to talk to X directly.