I did 2 shelves, 2 singles, and 2 doubles in mine.
From top to bottom:
1. Top slot I have a shelf. Since a drawer wont fit there anyway (due to the temp controller in the back corner), I didn't want to loose the space. On this shelf I have some beads and a digital hygrometer.
2. Single drawer
3. Singe drawer
4. Double drawer
5. Double drawer
This takes up a little over half the unit leaving the rest of the space on bottom for box storage.
6. Bottom slot has another shelf to keep the boxes off the floor of the unit. The space under the shelf is also a good spot for some more beads.
Can't really comment too much on single vs. double because I havn't gotten my drawers yet...the shelves are working good though...lol
You may have read my build thread already, but if not...here's mine:
http://www.botl.org/community/forums/showthread.php?t=44149