I have been using devastators judiciously, naturally.
They do exactly as described. If you are pursuing a starship heavy game, they provide the heavy "push" to knock out enemy starships.
In waves, while your forces are bogged down in the fleet brawls, the devastators do a good job of preventing starships from slipping through the initial frontline and thus attack your cmd / shields.
While the rarely get opportunities to fulfill their initial idea of piercing shields, their "anti big things" firepower is great.
It fulfills its role wonderfully. You sometimes need to knock out that _one_ annoying thing under a heavily guarded shield that a raider cannot (ion cannon, etc). In that case, if the devastator can get to it, it can be knocked out. For the rest of the time? You want a long range vessel to knock out those durable enemy units so your fleets can focus on the other fleet.
The only real weakness of the unit is actually a symptom of something else: The AI missile post is OP compared to other standard guardposts. It hits a wide variety of units, including its counter, really, really hard at a very wide long range and it is not particularly fragile. I say it is OP because the moment I enter any system the post starts its awful attrition on my most expensive units, and the unit that can counter it.