I have used one since 2012. Have the Oade concert mod, which Doug recommended when I told him I use AKG 460b's and naiant ck61's. (as opposed to the warm mod or T mod)
Pretty reliable device. You have to make sure to use recommended SD cards, but there are plenty to choose from on their list.
Marantz claims you void the warranty of you use external batteries, but I'm supposing most of us do!