Don't think twice, get it.
It is better than I expected, 'silicone' is more about the look than the feeling. It is hard plastic not silicone. Good texture and very confortable to use, makes the 405m a much better device.
If you got your 405m because of the aluminum premium feel, this sleeve is going to downgrade the premium looks, but that's being picky, and it's because it doesn't match the device color, if it did I think this wouldn't not be an issue.
Sound on the other hand changes, the crisp sound becomes hollow I don't know how to explain this I don't know enough about sound. Again, not a deal breaker AT ALL but this does indeed changes something.