I have played for an extremely limited time with mercenaries, and I do like what I see. In particular, most of the mercs cost a resource to use, which is great because prior to mercs any resource I acquired early game I would happily trade away to someone for a tech (since I wont use the resource on the tiny ships that are available to me early). But now? Trading away a resource not only deprives me of buying a nice ship early game, but it enables my trading partner to buy one. Now I think I would be more inclined to trade FOR a resource.
This is a good balance and makes trading decisions much more interesting.