If a Mormon says that to you it is their way of claiming higher moral ground and superiority. It is a judgmental put-down.
If anyone else says it to you it is a very high compliment indeed. It means you exude sincerity and are fun to be around and don't have a stick up your butt.
I heard my supervisor telling the new never mormon manager that so and so is the furtherest thing you could be from being LDS. I thought: Hmmm...I like to meet that person and shake her hand!