I've used it for a few shots. It can certainly be used, but you'll find f/4 too slow after a while.
On a 6D it might still be ok cause you can push the ISO.
Here's some of mine where I used it with a 40D:
http://www.iceinspace.com.au/forum/s...d.php?t=108322
http://www.iceinspace.com.au/forum/s...ad.php?t=93507