These won't last long and I guess you'll get access to the business lounges
Cairns - Osaka (Narita) 6 May - 19 Jun 2014 $499
Melbourne - Bangkok 4 Feb - 20 Mar 2014 02 $599
Gold Coast - Tokyo 6 May - 19 Jun 2014 $649
Sydney - Bali (Denpasar) 4 Feb - 20 Mar 2014 $699
Sydney - Honolulu 6 May - 19 Jun 2014 $899
There are a lot of dates / departures / arrivals in the link.
The sale fares don't qualify for business lounge access nor points. You need to add on the bundle for that.
$500 extra for lounge and points. Woof!
Here's the actual link listing the fares on offer
http://www.jetstar.com/au/en/special-offers#/?from=&to=&date…
Some sample fares & dates
Cairns - Osaka (Narita) 6 May - 19 Jun 2014 $499
Melbourne - Bangkok 4 Feb - 20 Mar 2014 02 $599
Gold Coast - Tokyo 6 May - 19 Jun 2014 $649
Sydney - Bali (Denpasar) 4 Feb - 20 Mar 2014 $699
Sydney - Honolulu 6 May - 19 Jun 2014 $899
IMHO there's not much between Jetstar Business & normal airline economy (Singapore, Emirates, MAS, etc)
so I'd save & use the later.
Wow, that's not that cheap. I thought that was return prices before I clicked on the link. It's certainly not worth paying that much for these seats. Better off buy full service airline economy seats as you say, or if you buy economy jet star tickets, if they haven't sold the business class seats they will often offer a cheap upgrade closer to the departure date
Here's a review about Jetstar Business Class with some handy info and photos that might assist with making your decision.
http://www.ausbt.com.au/review-jetstar-business-class-melbou…
Keep in mind that these flights are cheaper than the usual business class fares.
Narita is Tokyo, Osaka is KIX kansai international airport.
Examples of what is on offer and what makes this a bargain as opposed to random advertising?