I have yet to see anyone even use them. I haven't used them except when I played against the AI, when I first got this game. They only do 20 or 23 planetary damage, have weak shields and armor, and are quite pricey. It seems like you would have to devote too much resources to buy enough to make them useful. Even then, they are too easy to kill.
From what I have been reading, it seems like they had a pretty big nerf recently. I don't think making them expensive with a high fleet supply is a problem, but the way they are currently just doesn't seem to work. I would like to see them be more of a heavy planetary bomber that does more planetary damage and is quite a bit tougher than the current version. Only then, or by making them cheaper, can I see them being viable.
Any thoughts from you vets who know more than I?
P.S. I know there have been some discussions about sieges but I didn't see any directly talking about if they are useful for what they are intended to do, planetary bombardment. Hence, I made this posting.