We travelled one-way from Greymouth to Christchurch, with a two-night stopover at Arthurs Pass.
We were lucky with good weather & excellent visibility. The train was on-time, and had lots of free spaces, so was very relaxed & comfortable. Our heavy bags were checked in, and handled by the train staff.
We booked the "standard" fare
Because the train was almost empty, we could get great views from the viewing car, but take care that that if the train is more full, it might be near impossible to use the viewing car. Consider a "VIP" ticket at busy times, as there are more viewing cars per passenger seats.
The commentary through the headphones was clear, interesting & useful, but of course it does not work when away from your seat !
If you break the journey at Arthurs Pass, be aware that the station there has a shelter, but is not manned and has no other faculties. There is a visitor centre a couple of 100 meters away, which is warmer & comfortable if you need to wait.
Overall, the journey was a real highlight of our trip and highly recommended.