We did this one way from Greymouth (where we returned the rental car at the station) to Christchurch and it was simply magnificent - over 4 hours of non-stop stunning scenery and the carriages themselves (new in Nov.2012) are very comfortable and excellent for viewing with part-glass roofs. Even so there are two open observation cars as well. Your bags are checked in and go separately, so you don't even have to bother about them.
Refreshment is available on board and an audio commentary is supplied via headphones. (However, one irritation is that this does get overridden by various announcements. You'd think that the announcements could be timed so that they're made when the commentary is quiet or they could even pause the commentary.)The staff were informative and helpful and on our journey the train was only about half full so you could cross from side to side of the carriage for the best views. Simply brilliant.
Be aware though that, as strangely seems to be the case in one or two places in NZ, you can be charged more if you're a visitor. If you book through the website from outside NZ you'll only be offered the most expensive ticket (NZ$189 fully flexible), but if you ring them up you'll have the choice of all 3 price bands, so NZ$149 & 119 as well, albeit with less and no flexibility respectively. If, as we did, you know when you want to travel then this is a no-brainer.
To echo a previous reviewer...DO IT
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