For me "Mirror Weights Ideal" is a godsend. This is because Maya has a real problem when it comes to mirroring skin weights - it just won't mirror anything on the other side of the centre line including the centre line itself. I thought that this may have been fixed up in Maya 8.0, but alas no. So I was pleased to try Mirror Weights Ideal, and I'm happy to say it works nicely. Although it has crashed on me a couple of times due to overlapping meshes (I think) and possibly influence objects, I use it on just those areas where the mirroring is needed most. This stopped the crashing and sped up the process too. I couldn't find any other script out there that does what this one does, so it's a much needed addition to my arsenal.
