There are many synonyms for the phrase 'are impressive', depending on what is being referred to. Some alternative phrases to describe something that is impressive could be 'astonishing', 'awe-inspiring', 'stunning', 'remarkable', 'mind-blowing', 'spectacular', 'jaw-dropping', 'incredible', 'remarkable', 'striking', 'outstanding', 'breathtaking', 'splendid', 'extraordinary', and 'marvelous'. It is essential to choose the exact synonym that best brings out the essence of what is being described; therefore, it is wise to choose a synonym that truly elucidates what's being referred to, so that readers/customers can get the right impression.