I've been working in this plugin for a few weeks in my free time, but I just can't make it work stable.
After a lot of combinations this is the most stable version so I'm releasing it as a beta.
It may be somewhat useful and I would appreciate some feedback too.

It may still be unstable so save your scene before using it !

The script version seems to be more stable. It hasn't crashed my SI yet. So I'm also including a script version inside the Addon Folder that should be something like this:
C:\Users\myara\Autodesk\Softimage_2012\Addons\mSymm\Application\Plugins

mSymm2_script_Xneg and mSymm2_script_Xpos

They are basicaly the same with different options, one to mirror to -x and the other to +x.
Open them with an editor if you want to change something and execute it from your script editor or create a button in your main shelf. These are the most stable versions so if you have problems with the PPG and still want to use it, give these a try.

http://myara.web.fc2.com/mSymm2.html



How to execute it:
You'll find a "Symm 2.0" button in your Model / Modify / Poly.Mesh menu

Version Compatibility
v7.0 and above

Warning :
This plugins deletes half of the model, applies symmetry polygon, and then mirror weights.
Therefore, you may lose some cluster based constrains, index based shape deformers, operators, etc.

Interesting. Any specific "known"/reproducible behavior that makes it crash?

So far I haven't been able to isolate the problem because SI doesn't give me any error message when it crashes. In the previous "alpha" versions it used to crash SI with the 3rd or 4th click while i could run the script version 20 times without problems.
In this version it seems more stable surviving my "stress tests" (clicking it as much as possible). But still I can't guarantee 100% stability.

I've updated the plugin (let's say beta 2), now it seems more stable even with commands (now you can undo).

I'm not 100% sure but it seems that if you use the automatic "Mirror All UVs", the probabilities that SI crashes can increment exponentially. Without "Mirror All UVs" I haven't had a single crash yet but I'll keep testing it.

cool, - thanks for your effort ! Good for the next modeling session

beta4 released. It seems quite stable now

I'll try it soon. Right now I'm caught into some non-modeling/texturing stuff, but I'll come back with some feedback after I'll manage to do a few tests.

Edit1:
It crashed in a few mins of using. I don't know how to reproduce it yet.
If I may make a suggestion: mirroring along Z and Y could be helpful in some situations.

Damn, so it still unstable...! It hasn't crashed yet here, but I'll keep testing. Was it the beta4 version?
Thanks for your feedback I really appreciate it.

I don't know If I'll ever need them but yes I was thinking in Mirror X and Y too. I'll add that too if I ever get the Mirror X function stable.

Seems really nice! I will test tomorrow and give you some feedback. Thanks for sharing

Thanks. In the beta 4 I was focusing only in the main symmetry tool and it was stable (I think), but there were still problems with the second tab functions. I think I got it stable and working in my beta 5 (updated 2 days ago).

There are still some minor bugs, meaning it's not working like I want sometimes but at least it isn't crashing SI.
Now I can finally start to polish the code and hopefully release a final version soon.