Use them all the time. Required for mailing legal documents. Too bad can't run this strip-off seal type through a laser printer. It gets too hot and diminishes it's effectiveness.
I use these to mail my new children's book which is 8x10 but only 24 pages. The envelopes are easy to use and I often color all over them with paint markers. The paper holds the thick ink really well. Only thing I'd do differently in the future is spend the money on the sticky-seal just to make my job easier :-)
As a freelance photographer I use these envelopes to hold the smaller portion of my portrait packages when delivering them to clients. They hold up very well as well as protect and conceal their contents. I cannot comment on how well they seal as I do not use that feature. These envelopes are perfect for my use.