As a Reach Out! Youth Ambassador, I travel between Adelaide, Melbourne and Sydney quite a lot, and all those time, I am always staying with YHA hostel. What I love about YHA is that they're cheap and they always have a lot of residents, thus you are more likely to meet other tourists or backpackers and I love talking to these people, and exchange experiences!
Most of all, YHA is very clean and yet, affordable. I wouldn't say it's cheap because as compared to some other hotels, it's definitely affordable, but if compared to other hostels, for the amount I'm paying, I'd rather stay in YHA!
They also provide free breakfast, and they have a huge lounge for you to hand out, and a pool table, oh, and also computers for you to get online!