First, I want to say that I really like this plugin. Except the bug decribed below, it works fine.
When I enable synchi in posts/pages editor, it opens HTML tab, but TinyMCE is still shown below. On switching to Visual, everything is fine again, also on opening HTML tab again. It occurs only on "startup".
(I use Chrome.)
A thing I would really like in synchi, is to implement something like an image viewer, so I can take a look at the themes/plugins image files.