I wouldn't recommend this envelope for mailing secure documents only because of the fact that I believe the sealing strip on the envelope is a little too easy to reopen once the envelope has been sealed. Otherwise for everyday mailing it is quick and convenient!
Works well for everyday. I use these envelopes to mail checks and for balloting. Make sure you cover the check value as they may not completely hide the contents. I find one thickness of paper on the written side of a check is sufficient.