I've used Nabtrade for several years for the long term buy & hold component of my portfolio, and found it to be generally fine. The only times I recall it going down was when everything else went down due to some failure at the ASX or Telstra. Though given that I do < 10 trades a year in it (I do all my short term trading thru IB) maybe I just didn't notice. It has the convenience of a linked high interest account for stashing dry powder, from which funds can be instantly transferred to the trading account when needed, and the interest rates on that linked account were more than competitive with those of the other CHESS brokers back when I was looking for a broker for my buy & hold arm (currently paying 4% but they have not adjusted for this week's hike yet).
Getting options trading enabled for my Nabtrade account (just in case I needed it for whatever reason, I would not recommend frequent options trading thru Nabtrade, it's a ripoff, like options trading on every other CHESS broker) was, however, a rather onerous and frustrating process, which I've already had a rant about in some other thread in the derivatives forum, so I'll refrain from ranting about it again here.