The only reason I do use them is I'm with Kaiser and they've (finally) put me back onto brand rather than generic. And if they screw up or are late I'm only 9 miles from the facility, so it's really worked well. I don't know if I'd use mail order otherwise.
I use Kaiser too, and they have always gotten my meds to me. I order as soon as I can, and I know that if they screw up I can go to the regular pharmacy the next day. Ordering my trileptal is something I just remembered to do, too!
Another thing to sort of...I don't know, "ensure" that you never run out is to dose out a week's worth of all of your meds and put it into a safe place! That way if it's, like, midnight/Saturday or Sunday and you have the wrong meds in the mail you can get through til their local pharmacies are open again. Just an idea!
Plus, if you can't drive/don't feel safe going out alone mail-order is a Godsend...<3