I've got them on front & rear. Find them great most of the time, I've found they do tend to slide, but it is a nice predictable slide. As said above, it gets interesting/fun in the wet mud/roots/rocks.
I'm looking to change the front for a different type and stick with it on the rear. Especially after Ashton Court on Friday, front washed out (i couldn't see my front tyre, only mud), knee's hit the rocks, lost my garmin and now have a bad neck. Found the garmin after 20 mins of searching, and now can't ride until the weekend. I blame the tyre, not my rubbish skills. 🙂