Done the HK-SH route. Soft sleeper is very nice for the money. Bunk bed, chair, separate toilet and sink. Very comfortable although sharing with a stranger would be awkward and, given some Chinese habits, potentially annoying. Life outside the cabin - dining car for example - was seriously underwhelming so bring plenty of food.
Done hard sleeper from BJ - SH and BJ to Urumqi. Wouldn't do it again, but if you are young and adventurous then why not? Was a great experience. Just watch those toilets!