In contrast to the other classes, there is no limit on how many people will travel. Even though there are reservations one is supposed to move together, so that as many people as possible are able to sit. A good way to spend time on the train is to go to the dining car. Meals are cheap and usually ok. Especially when you're travelling hard seat, you'll be happy to have more space.
Trains are usually very punctual and safe.
At some place there is a black market for Chinese price tickets. The worst that can happen to you is that the train personnel doesn't believe you're a foreing student studying Chinese in China. Tickets are sometimes quite limited, so the best thing is to book your next ticket at the same day you arrive at some place. If you stay at one place for a couple of days don't forget to book at least two to three days before your planned departure. Many stations, especially the big cities, have special booths for foreigners.