look here at this great post from someone who has been through the process and worked out what the issues with mesh are

all the text below is from the link above at Lorelei Armstrong website and he makes some very valid points about the new mesh features of SL and the crippled limitations, the text below in bold is my respsonse to the issues as a person who works in the 3D realm content :

Mesh Problems.
Mesh is not a perfect world. Some of the problems relate to the fact that it is so new and its capacities will grow with time. For instance, at the moment mesh avatar creators are confined to the armature (skeleton) underlying the basic SL avatar. This skeleton is manipulable to a degree, but if you want to be a mesh spider you may have to wait a while.

Other issues:

1. Adding a script to a mesh object will make the prim count explode. This is based on the fact that a script places a heavy burden on SL’s servers. This is true of any scripted object. The controversy that has arisen is that LL chooses not to represent this burden when the scripted object is made of regular prims and/or sculpties, but have chosen to burden mesh. Weighting mesh in this way will likely kill the possibility of certain mesh vehicles and many, many interesting mesh builds and devices.

There is no reason for this ? its effect will limit mesh use

2. The larger the mesh, the higher the prim count. This is another rather dreadful choice from LL, and is a terrific disappointment to building and tree creators. We finally have a prim size increase to 64m and gorgeous big builds are going to be punished. There may be a balance to be had, cutting larger mesh objects into smaller pieces and finding a prim count “sweet spot,” but the purpose of having larger prims doesn’t apply to mesh so much as creators may have hoped.

as per usual this is going to create more buggering around on creators parts unecessarily, prim size has very little effect on server cost or render time, meshes that are used should be the same prim cost whether 4m or 64m in size, again no proper reasoning behind this.

3. The great ability to make a mesh have physical properties, like being able to walk through doorways and walk on floors? Right, that increases the prim count. Want to keep the prim count down? Don’t define a physics shape for your mesh, set your mesh to phantom, and define the physics shape with regular prims (get a good 100% transparent alpha texture— you’ll need it).

Without proper physics cages associated for hardly any extra cost whats the point ? the whole problem with sculpty already is no proper physics or collision.

4. Mesh is one-sided. Imagine looking inside a shape that has a regular texture on the outside and a 100% alpha on the inside. This is due to something called “normals” that tell the system which way light is reflecting off an object and yielding an image. At present they can only go one way. If you want two sides, you have to make two sides. This, of course, increases the prim count.

two sided is more realistic to the physical world but not providing two sided normals means prim cost will be less efficient for lots of things

5. Linking increases prim count. This one is rather infuriating, because mesh is set up to practically require linking to regular prims (see #3). But link a mesh into your regular prim build and watch the prim count of the whole thing go up. Building creators will likely be stuck having to use packaging systems, just as they do when using phantom prims in a build. However, see #1. Rezzing a compound object with the rezzing script in the mesh for final positioning may cause a temporary prim count so high that your customer can’t rez the build.

linking to other prims im not sure why that should increase prim beyond the prim count of the two objects themselves ?

5. Mesh cannot be flexible.

nice feature to have but points 1-4 are more important

6. Mesh avatar faces do not animate.

nice feature to have but points 1-4 are more important

7. Creating mesh requires use of an outside 3D modeling program. I use Blender. Learning Blender is *#&$@&$*@#.

do decent sculptys already ? then that isnt an issue

8. Turning to the very dark side, the potential for vast quantities of mesh to be uploaded into SL from outside libraries is huge. There might be massive intellectual property law violation ahead. Yes, you have to have payment information on file with SL before you upload mesh (so they know who you are), and you have to pass a dorky little quiz about IP issues written by LL’s lawyers to protect them from exposure before you can upload mesh (hint— you can’t fail the quiz). But there are an astonishing number of ways around these flimsy barriers. And that’s not to mention the copybot exploits within SL itself, which are an unaddressed plague.

that will have to be content owner based policing as is currently the case

*Don’t bother trying to upload a mesh at a tiny scale and then scaling up. Doesn’t work. But do be ready to see the prim count of your mesh increase as it grows larger.
Posted by Lorelei Armstrong at 12:59 PM