They're worthless piles of shiny crap. For all the trouble they must have gone through to add them back into the game, they are so nerfed it's pathetic.
Let's see:
You have to research them, which is very very expensive and time consuming.
You have to build them. (no big getting 6 or so constructors)
You then have to wait 10 freaking turns to do anything? This is what bugged the $$$$ out of me.
Then you blow up the system. Cool.
Then you have to spend ~15 turns to get to another system. Again, retarded.
For all that effort, it is ALWAYS easier to just build transports and take the system. For terror stars to be of any real value this is the variable that has to change. There has to be a reason why a terror star is really better. Perhaps if you have a horrible soldiering race it might work, but then you could just core detonate right?
To me the better way would have been to allow you to design them, and build them like normal ships, but put a speed limit on them of say 4. (then with bonuses they'd go like 8) Then make the attack take 1 full turn. (you have to have full moves to attack, and then your done.)
Anyway, like 90% of this game TS's were balanced for playing tiny or small maps. So I suppose if that's the style you play they might have a little more use.
Also, avoiding the enemy ships is a non-arguement. If the AI had any brains it would de-orbit and attack. Also the AI doesn't build good ships anyway, so they're not a problem either way.