I missed out on the previous Amazon deal for the same price just a few days ago - https://www.ozbargain.com.au/node/771431
So when I saw the same deal at Snowy's I jumped on it.
Sea to Summit states this liner adding an extra 15°C of warmth, but in reality, its maybe a few more °C of warmth from what reviews are saying. But still a worthwhile upgrade for a cold night out camping/hiking, more comfortable, plus it will keep your expensive sleeping bag clean!
I've got the 8c one and its great. Not really sure if its adds to the warmth, but its much more comfortable having the soft fabric against my skin than the synthetic sleeping bag. Also much easier to wash these than the sleeping bag.