This was my introduction to Chinese trains, and the hard-sleeper berth was much more comfortable than I had thought it might be. In retrospect, that's because the trains from Hong Kong attract a different sort of clientele, because the rolling stock on that route is much more luxurious than most Chinese trains, and because getting tickets was simple and orderly.
Customs on the train was simple, as you stay in your car and the officials come to you. The train doesn't leave Guangzhou station until after 6:00, which means there isn't a lot of Chinese scenery to see before it gets dark.
The train arrives in Shanghai around 10:00 am. The last hour or so (i.e., everything since I woke up) is very urban. I arrived in Shanghai well rested.