To add to other comments:
- This wand is much more useful when upgraded, since you get more powerful wards/sentries.
- The first three zaps produce wards, which shoot a specific number of times.
- The next three zaps produce sentries, which will zap once every two turns until killed.
- You can place wards anywhere you have vision, even across walls.
I've tried playing with a staff of warding once or twice, but I don't think I've managed to win yet. It's a fun wand but it requires planning and careful management of charges. One thing I enjoyed doing was placing a couple of sentries in a spot that mob patrols would frequent. No more random mobs to worry about!