JetStar is the budget subsidiary of Qantas, and one of several low-fare, no-frills airlines that operate in Australia and Oceania. Although JetStar has its own livery, separate ticket desks at the airports and own planes, the Qantas website shows the regular Qantas flights as well as JetStar flights and is thus the best place to look for connections and fares.
The main competitors of Jetstar/Qantas in Australia are VirginBlue and TigerAirlines. However, if you are looking for flights at a short notice, and if you are travelling with any hold luggage, a JetStar or a Qantas flight is likely to be your best bet: although VirginBlue can offer very good prices, these usually have to be booked weeks in advance, while TigerAirlines have a complex and not very transparent fare structure with high penalties for hold luggage. Additionally, Qantas is a national carrier and thus, together with JetStar, has the highest coverage.
The prices we paid were not particularly low (at least not by European cheap flight standards): we paid about 280 A
We took two JetStar flights, one from Cairns to Darwin, which we booked online couple of days before departure; and one from Darwin to Melbourne which we booked at the airport on the day.
I can't fault the members of JetStar staff I encountered: they were friendly, polite and helpful, and particularly the ticket desk person in Darwin was very understanding of the predicament we were in at the time.
The Cairns-Darwin flight continued to Singapore, and thus was treated as an international flight: this meant we had to check-in earlier but I suspect it also was the reason for a more comfortable seats: overall JetStar seats were not too bad in either of the planes, fairly wide and well padded in black leather (much nicer than for example Emirates Economy seats we had to endure for the total of about 60 hours flight time), although as usual in Economy, they could do with a bit more leg room. Comparing to the European cheap flight king Ryanair, JetStar had both bigger seats and more leg room.
However, the seats on the Darwin to Melbourne flight did not recline at all: considering that it was an entirely a night-time flight (it left at around 2am and arrived around 7am), this lack of recline was simply awful and made what could have been a reasonable flight into a gruelling one (I for once cannot sleep completely upright).
All in all, JetStar flights we took were OK: pretty much what you would expect from a non-frills airline, lower key than some European budget carriers (Ryanair or EasyJet) and with quite reasonable seats (which is, after all, the most important thing in a flight). If you are contemplating a night-time flight with JetStar, do check if the aircraft you'll be flying on has reclining seats – or prepare to be sitting upright all the way.