When Jetstar Int was launched I did a quick comparison of MEL to BKK fares, between JQ and TG. The fares were roughly the same, but that was with no frills ordered on Jetstar. Also if you have a connection ex BKK, if you travel with Jetstar you have to overnight.
The only way I can see Jetstar being good value for anyone, is if you get a really cheap seat in one of their sales. A Thai friend of mine got one of their launch fares for about $300-400 return MEL-BKK. This is hard to beat, but on their everyday fares there is better value for money out there.