I got a 6 month Telstra Simm a couple of months ago for $100 (down from $150) and its expiry was 12 months away so planning to load up on these to top it up when it expires.
I was on Post Pay with Telstra at $65 pm so these have been a huge saving for me.
I also believe unused data rolls over to DataBank.
I hate 28 Day plans. They get 13 Payment out of you a year. They are a Con