I’ve been thinking of various ways to build optimized mesh based on Zbrush meshes for quite some time but now I think I know the best possible solution.
These tips are mainly for max6 and I don’t know how well they will work on earlier versions.
First make highres model in zbrush with most important features and export it to max. Add optimize modifier with face treshold 4-10 and then add multires, generate and adjust the slider so you have adequate viewport performance (optimize modifier is there just to reduce the memory usage of multires). Then just convert to editable poly.
The hardest part is to actually build the optimized mesh. You can use all polytools that will add their result to edgeselection but I mainly use cut and slice (and quickslice of course). So we begin cutting the wanted edgeloops with cutting tool. You can ignore all existing poly edges and jsut merely cut where you want (it is important that you are using edit poly because the cutting tool in edit mesh won’t work properly). Right now the mesh dont have to look good. You can and should cut lines so they overlap each othes sligthly so you can be sure you get crossing section. After the mesh (edge selection) is ready, extrude it a bit, extrusion base width 0 and height about 0,1. Grow selection once, select invert (ctrl+I), convert to vertex selection, collapse, remove. The mesh is now looking better but it has many extra vertices that need to be removed. Max doesn’t have native method to do proper selection so we use a script called “select by stats” created by Externe at CGtalk. You can get it here. Just run the script and add (customize UI)the utility it created (main UI, select by stats) to a shortcut or toolbarbutton. select all vertices connecting 2 edges and hit remove. The mesh looks much better now though it may have some parts that need manual fixing. That’s pretty much it. You should experiment a bit with this before you start working on larger objects. It is important that you don’t loose the edgeselection at any point as it is the point of the whole thing. I may be wise to save named selection sets every now and then if your model is very complex.
Another cool trick is to use conform compound object. you can use the cut tool trick to create simple mesh, add push modifier, collapse and make an instance of the object. Apply conform, set along vertex normals, set suitable standoff distance 0-1 mostly and hide wrap to object. Select copy method and pick the Zbrush model from the scene. Other instance of the object should now be surrounding the Zbrush model and the other wrapped to it. You can now edit the surrounding object and the changes efdfect the wrapped one. The whole point in this is that you only need to do rough edits on the mesh and they’ll automaticly take the right shape.
Basicly you could make a ready, displaced model with Zbrush, optimize it in max and bake the displacement map with external program (like ati normalmapper or ORB) so you don’t need to do the work in Zbrush twice.
If you don’t understand something, ask and I try to clarify. These tricks have helped me alot so I thought I might share them. Hope you like them. 
.