From New Zealand they are definitely good value: the fares get more expensive in a few months. But they do come with conditions attached that vary from airline to airline, eg how much do they charge to change the return date etc. And you probably have to pay for them quite soon after booking, which means you won't be earning interest on the money in the bank. I booked in November for an April departure which wasn't so far ahead, but September is still quite a way away...
I used a Singapore Airlines Earlybird flight. I wasn't sure how long it would take me to walk so I wanted a fare that had no charge for a first change of return date. (Some airlines charge $100 or $200 or more.) The only problem I then encountered was that I wanted to get out of Paris earlier than I had booked, at peak holiday time in July! But the service I got from Singapore Airlines was excellent in terms of helping me to get waitlisted in Paris, and then in terms of courtesy etc when I turned up at the airport to try standby on a flight waitlisting hadn't 'worked' for.
Margaret