Once you have a base with all gold mines maxed and two factories (one producing dirt cheap commons and one producing power kits), it works very nicely indeed, particularly for maxing out your mechs. As has been said, it's a slog getting there, but if you do, I think it's better than paying for silver boxes. Bear in mind that when collecting items from the factories, you usually get a "watch ad for reward", and the reward is an extra item, which is guaranteed rare but has maybe 25% chance of being epic. I don't use the top end of the factory for legendaries, as the drop rate doesn't seem much better than free fortune boxes and it's expensive, but I might be doing that wrong.