Optus recently improved their pre-paid roaming and it seems to best almost all of the travel sim options going around, especially for convenience and country coverage. Buying multiple packs would net you a fair chunk of data too ($40 for 20gb for instance). The only caveat would be that you need to activate it in Aus first.
$20 Data Roaming
Up to $210 credit to use on data. At $0.02/MB that gives you 10GB
Available plan:
Prepaid Epic Data, Prepaid Epic Value, My Prepaid Ultra, My Prepaid Ultra Plus, My Prepaid Ultimate, Prepaid Ultimate Plus, My Prepaid Long Expiry, Optus Prepaid Long Expiry.
Available Destinations:
Mainland China, Hong Kong, Indonesia, Japan, Malaysia, New Zealand, Singapore, Thailand, UK and USA.
Expiry:
14 days
*For the cheapest method on activating a new sim, try this post - https://www.ozbargain.com.au/comment/7969742/redir (but this sim instead https://offer.optus.com.au/shop/prepaid/sim-card/2-sim)
Damn, could have really used that when I was just in Japan for the World Cup.