The actual sealing strip doesn't hold very well at all. I had a mailing of 500 and had to set each envelope under something heavy to make sure it stuck. Easy to do and you don't have to use your tongue (hurray) but the sealing adhesive needs to be better.
These envelopes are extremely easy to use and the quality is the best available ANYWHERE (and at any price!). There are "cheaper ones out there but the quality and what the cheap paper stock says about it's content is vitaly important to small and large businesses alike. Image matters and this is everything.
Good enough for mailing bill payments and similar. Unfortunately they are not "nicer" enough to double as correspondence envelopes. Also the self-adhesive material is not strong enough; we always have to use a sticker or tape be sure envelope will closed in transit. (hint-hint)