I do worry a bit about the garbage created by the peel off strips (are they recyclable paper or some other material?) but when you have a lot of letters to send it sure is easier than licking all those envelopes.
Every time I've put something in one of these envelopes it comes unsealed by the end of the day. Thank goodness I'm a procrastinator or I might have continued to send envelopes that open by the time they reach their destination. Do not buy. Get anything else!