Pheonix's performance is brilliant. He has to carry so much of the film just with his facial expressions, bringing to life what Johansson is saying through his reactions. Her vocal performance is similarly great, she gives presence to a disembodied character and engages you in the story of her development.
I wish I could encapsulate the feeling of beauty, heartbreak and sheer wonder that Her manages to convey, because that would be the best way I could recommend it to you. As an attempt, I've written this haiku:
She has no body,
And so she is evolving
Beyond mere beauty.
There are no explosions, no car chases, and a complete lack of Lego figures (shame) but this film really touched my heart. Take hankies and allow time for a cup of tea or two after viewing.