One constant "gotcha" in the Carrara-to-Poser pipeline is surface normals.  For some reason, modeling workflows that include duplicating, or duplicating and welding halves, will cause Poser to decide that certain groups of faces "must" be facing the other way.

There seems no reliable way to straighten out the issue in Carrara itself.  Reversing normals in Carrara doesn't fix the problem for Poser, even if you make sure to detach polygons, et al.  The simplest solution is, oddly enough, to open the mesh in Poser and use Poser's Grouping Tool to find and swap the errant section.  Then re-export the fixed item from Poser.

(Since this is part of my normal flow anyhow; model in Carrara at arbitrary scale, import to Poser using scaling, then scale and position to fit in the Poser universe before exporting the properly sized and fit object, I don't find it onerous to export for a surface normals problem as well.)

To agree with the other posters; Poser will now deal with n-gons, but it renders them badly.  The crease angle set in Poser also does not fix all of Firefly's problems with non-organic shapes.  I have found it nessary to add an additional "constraint" edge loop near some beveled edges in order to keep them from marshmellowing out just enough to make a non-flat surface.  Related to this is the "bagpipe" issue; Firefly smoothing gets a little nutty around cylinders with a length-to-diameter ratio of larger than 1.  They will often bloat out, ending up looking like a python that swallowed a gopher.  A constraining edge loop or two along the length of the cylinder controls this.

For figures and clothing, tris will crunch worse around joints.  Quads are superior.  But when quads get too non-planar Poser will often render surface artifacts.

On the Carrara side, a few more gotcha's; Carrara leaves behind the lathe profile, as stated above.  It also does not always weld the close of a lathed object properly.  It is worth selecting the entire model and hitting "weld" with an extremely low welding tolerance to catch any failed seams.

Carrara will eat groups -- make sure to save group information by, say, changing named polygon groups to materials groups instead.  You can change them back later.

Carrara does not work well at Poser scale, and does not scale and unscale accurately enough to use that as a trick (if you try, say, importing from Poser at scale, expanding 100 times, then reducing 100 times before export, the mesh will shift slightly, usually in the x of the Poser universe).  It seems to make for less headaches to work at a scale Carrara finds comfortable, and fix the scaling and position of the mesh in Poser.

Carrara defaults to spherical mapping when creating vertex objects (the spline objects, however, are created with a decent cylinder map.)  Carrara also defaults to "wrapping" UV's, which wraps them across the edges of the UV space.  This is only appropriate when a single surface uses the entire UVspace: most Poser creations have more complex UV maps with several different shapes sharing a single UV space.  If, however, you uncheck "wrap" UV's, Carrara does not actually unwrap them.  What it does, is invisibly wrap them BEHIND the displayed mesh.  You can discover them by grabbing an edge in the UV mapping room and dragging it.  To properly map a cylinder in the Carrara UV mapping room it is necessary to detach a row of polygons, drag it around manually, then merge it on the other side.  For a complex cylinder this can be a lengthly operation.  Using an external mapping application is probably wiser....especially since UVmapper Pro (Windows only) is so cheap.

All I can think of at the moment here.  At least Carrara doesn't offer the option of double-sided polys, which can cause some strange problems in Poser.