The reason angel in our language is 'angel' and derives from the Greek is because English itself is a jumble of languages and angel and clearly they saw a connection between the messenger and angels.
The original Hebrew word for an angel is 'Malak' and can be translated as messenger also, but also envoy, or ambassador. Personally, I prefer the latter, it makes sense in terms of the roles of angels in lore, of course they do have messenger roles also, but in biblical literature hey are usually working on God's behalf and are therefore ambassadors.
Demons were normally named by name, the closest Hebrew word for demon is Shedim, which has an interesting etymological history in it's own right; however, it is used as a reference to pagan gods also. Some people will reference the qliphoth, but in my opinion that is erroneous.
The obsession with demons is largely a Christian invention and in terms of an obsession with them in magic largely a Medieval Christian venture.
I work predominantly with angels, but not of the Hebrew variety, I have also worked with demons in the past. I clearly believe in them, but I make no conclusions as to what they are. My experience in terms of their nature is that each one is different and one person's experience can differ from another person's.
Have I seen one, not physically, astrally, and whilst in trance scrying in a crystal ball (which I class as astrally, others would disagree).