Use it with most recent version of C5.
It needs to be noted that it works great with Concrete5 5.5 and higher.
Use it with transparent pngs as well. Very good money well spent.
I encountered one problem though, when I used it with the splash theme, the backstretch block showed up even when I logged out of the site. Don´t know if this is rather a splash theme prob or a block prob
I didn´t report it or write for support as the landing page didn´t look good anyway. Anyway I love this block