A complete search of the internet has found these results:
I agree with you is the most popular phrase on the web.
Both phrases have the same meaning and usage. "I agree with you" is more colloquial and very commonly used. "I am in agreement with you" is used in more formal settings.
11,003,999 results on the web
I agree with you that everything seems to be getting worse in this town.
I know if might not seem like it, but I agree with you 100%.
Did I tell you that I agree with you about these decisions?
Oh, believe me--I agree with you that we need to do more to fix this place up.
I agree with you on the first point, but I disagree on the second.
I do not agree with you on this issue.
I think that I agree with you, for sure.
I do agree with you about some things some of the time, even if we disagree on most things most of the time.
1,049,877 results on the web
I am in agreement with you about how to move forward on this issue.
Did she tell you that I am in agreement with you?
I am in complete agreement with you on this decision, captain. We should definitely head east.
I am in agreement with your decision about how to deal with this mess we created.
Are you in agreement with the rest of the men onboard?
I am in agreement with you: we do need a new desk.
I said that I am in agreement with you about most of these topics.
He is in agreement with me that the new refrigerator is much nicer than the old one.
I am in agreement with you as usual.