I had blown-in cellulose (about 4" worth) and rolled out R-30 fiber on top of that.
Didn't really notice much difference in the one section of the house, but the upstairs was noticably cooler so I suppose I didn't hurt anything!
What's a good way to use the attic for storage if you have fiber insulation? I've got some boards sitting across for support of boxes and such. I know it's not maximizing the r-value to push it down like that, but what else could I do?