They were always cheaper than regular airmail letters, however after they were discontinued (around 2004, I think) you can only use them if you add postage up to the global rate. On the plus side you can now add things inside to increase the weight up to 1 oz (about 27 g I think) ... which is a good thing given what happened the first time I tried to send one to Eric. It looks like the sorting machines tried to eat it... and of course the machine ate the most important part, the 50.
I had one Chief Anderson stamp left, and added some card inside to make it sturdier (I hoped). It worked, and you can see how nicely version 2 traveled on Eric's blog.
Eva recently had a post about aerogrammes, (which generated more aerogrammes and another blogpost) so I sent her one, too, also with a bit extra inside. It arrived safely. No more chief stamps left, however I thought the butterfly worked as a symbol of flight.