I've got a full 1080p set and to be honest, it's difficult to give an answer to that question. You have to take into account your viewing distance and screen size - if you're 20 feet from a 42" 1080p set, you probably won't notice much difference between 720 and 1080.
I generally leech 1080p on films I want the best quality from - examples being Transformers (which is simply stunning in 1080p), 2001: A Space Odyssey, The Band of Brothers TV series, etc., whereas I grab 720p versions of older films and non-action orientated films. Although I got a 720p copy of Rambo 1 and it's hard to see how it could be much better picture quality wise in 1080p due to the original source.