'by' or 'with' are both fine.

Actually either "impressed by x" or "impressed with x" works in the example sentence. I haven't thought about it very deeply but I think "by" and "with" are interchangeable when used with "impressed".

My apologies. I guess you should use "WITH" "I’ve been very impressed with the organization and execution of the event, the level of interest and the quality of material presented in the past. I’m optimistic that this year’s meeting will be even better.” I read a sentence in this website: http://www.bluetechforum.com/testimonials/lorem-ipsum-dolor-sit-amet-consectetur-adipisicing/

You use "By" when you are impressed by something If someone impresses you, he or she impresses you with something. So in this sentence you should use: by I wasn't very impressed by the service... Hope it helps