Instant Meshes is OK. It's free and does a relatively decent job in some cases, but often the topology is still very dense after retopo with Instant Meshes and it's difficult to force the edge flow to generate in such a way as to be useful for animation or deformation. I would recommend retopology by hand in every case except for hard surfaces or static objects. There are some very good addons for "manual" retopology in Blender, but I will usually do my retopology in 3dCoat anyway.

If you really want to avoid doing manual retopo at all costs, then saving the base mesh could be a great solution. With some tweaking, you may be able to fit the sculpted mesh into the original base mesh cage and then snap it to the surface with some of the tools in blender for projection.

If that isn't possible, and you still need a good way to generate a base mesh automatically, I highly recommend using a product called Quad Remesher by a company called Exoside. They have a Blender plugin, and the results of the retopology using that algorithm are far superior to any other automatic retopology process I've seen so far.

This is not a direct link to a marketplace, so I feel comfortable providing you with this link for more information... Quad Remesher

In my opinion, Quad Remesher is an extremely valuable addon and tool for creating quality retopology that can actually be used on organic or deformable models, especially with symmetry.

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