Should I use "impressed with it" or "impressed by it"?

With it or by it?

For example, take these two sentences:

"I wasn't impressed with it."

"I wasn't impressed by it."

Which would be more formal/proper?

Also, say you wanted to have....

"I wasn't too impressed"

or

"I wasn't that impressed."

Which would be more formal/proper?

Both definitely sound fine to me as a native speaker, and I'd go so far as to say that they are both correct. However, I think there is a slight difference in meaning - "I wasn't impressed by it" looks rather a lot like our (English's) passive construction, and so the preposition "by" really underscores that the thing itself failed to impressed you. "Impressed with it" means more that you evaluated the thing and found it unimpressive. Most people, however, would be hard-pressed to decide which of these two options more suits what they would like to say, and so it is safe to use them interchangeably.