ORIGINAL: Raap
I rarely bother with them, since by the time I find them I usually have far better ships myself. I usually send them straight to the space port to dismantle them and get tech. I did have an interesting game where my early-game enemy managed to find two of them though. While my ships had 40'ish firepower( i.e. tech level 1 pretty much) those ship had like 400-500 or something and excellent shielding. As I was soundly beating him, I was suddenly surprisingly challenged when those ships arrived and added some much needed competition for my ships. Eventually had to combine all my fleets and ships to take them out in a single attack. Must have lost like half my ships there, but it won me the war.
I've found enough Titan Beam armed cruisers and frigates early on that I can literally wipe out the defenses of a home planet with the one fleet of 6 ships.
On the other hand, I've seen AI empires do the same with 1 or 2 of the really advanced ships as well. And you should really see what happens when an agressive AI race gets a world destroyer...ouchie!