These are being used to differentiate our mailings to be a little bit more upscale compared to the plain manila 9 x 12 envelopes we normally use. We like the pre-printed "First Class" on them and self-gummed flaps hold the contents inside the envelope securely. For roughly .40/envelope $USD it's worth the piece of mind and impression on the mailing recipient. Definitely recommend!
Envelopes work well for mailing card stock certificates with cover letters. Material of the envelopes isn't as thick as some other brands but still holds up well for mailing light weight content.
The adhesive on the self-adhesive strip seems different in this batch of envelopes. I find that the protective cover strip sticks to it when trying to remove it and can tear. Sometimes I have to remove the strip in a couple of tries / pieces.