I have 2 of my mounts filled. I filled the upper one by the valve cover with some left over clear RTV I had laying around, mostly out of curiosity, and it has held up well for about 2 years. I recently did my front lower mount with window weld. I am right at the point where stuff will start to rattle, but not there yet, which is exactly where I want to be.
From my experience with the two, I think I like the silicone RTV the best. It isn't nearly as messy, and seems to dry about as fast as the window weld. Plus, 3M window weld is fairly expensive. I say if you do go for it, just buy a large caulking gun sized tube of the silicone.
For the record, I have had mount inserts in the past in my civic. They were energy suspension brand I believe. I had the most annoying rattles on the planet after that install, and they seemed to get worse over time.