BigBen,
Very nice pic. I have a similar mountain render I created with some really nice cumulostratus (which I created by accident). I'll post it once I get home from work.
Also, were you using the 4 sided billboard object posted from a few weeks ago? Thats the one I'm using.
I've also been doing tons of tests will billboards. As you stated, it saves bigtime on RAM, but doesn't really speed up the rendering, or so I've found. I find it works best for objects that are very small and plentiful, like grasses (up close), or trees (far away).
I kinda wish the population options included X,Y, and Z rotations, which would help as well.