It can be hit and miss, but also depends on your preference in which class. All of them are viable, of course, but I enjoy AST the most.

Some months ago, I solo healed an e9 as a Noct AST after the other healer DC'd in the first minute of the fight. We didn't wipe, and it was stressful as all heck... But it was fun!

But I've also had many runs where it's smooth as butter through the run, and... Just wasn't as fun at the time? I'd watch one of the DPS and would wish that I'd gone in on BLM or something instead.