[mesmark]

I have been thinking about this each time I stop by.

It seems to me "with" has more of a feel of character or comparison where "by" has more of a naming the agent feeling. That doesn't make much sense, but I thought I'd join in on the topic.

I was impressed with him.
- he did a better job than I thought he could do or better than most would do


I was impressed by him.
- who impressed me? --> he did.


I was impressed with your lectures.
- they were better than I expected or better than most

I was impressed by your lectures.
- some data or information in the lecture was impressive


Anyway, in most cases they seem interchangeable to me. Those are just my thoughts on the nuances of the two.