The Missives are awesome.
Earlier today, got home from work, was almost capped on Garrison Resources.
I bought Missives for Mok'gol Outpost, Broken Precipice, and Shattrath Harbor.
35 minutes later, I had 2400 Apexis Crystals.
I'm almost capped on Garrison Resources again, and I've got follower missions that are going to bring in 1200GR tonight so I gotta buy some more missives and do em on Sat (you can buy multiples and bank them).