I hear "stressed out" more often too, but agree that it is informal. "The wedding preparations are putting a lot of stress on Mary" would be a more formal way to say it.
I don't think we use "stressed" by itself very much---it tends to be either "stressed out" or just the noun.
Oh my God, I don't think I can handle this stress much more! My sister-in-law is completely stressing out the whole family. My poor mother had to go to the doctor last week. She thought there was something wrong with her heart, but the doctor said stress can cause all her symptoms. He gave her a brochure about dealing with stressful situations.