I completed my 2011 census return this morning using the online form. It took about quarter of an hour and was a relatively painless experience. I was impressed with the online site; it was smartly presented and guided me through the required answers with ease. The only thing I might have wanted to see was the size limits for each field; perhaps a dynamic countdown (as found on most Twitter clients) would have been a good way to do that?
Overall though, an easy task, helped by a good website, and I look forward to hearing the initial analysis of how it paints the shape of 21st Century Britain.