I was an EMT instructor. While teaching a class on vital signs I had a student take my BP it was 220/130. What the hell give me that! Changed arms and took my own. Very close to the same reading. No warning, no signs, no symptoms--just silent death.
My ex wife died of a stroke cause by hypertension so this really got my attention. I was in the docs office the next morning! I am one of those people that will need meds the rest of my life including a diuretic to keep the BP where it should be.
I agree that most of the auto reading BP kits are suspect, if not outright garbage. Now having said that, I carry a wrist monitor in my jump bag. Does it gives me an accurate reading? Maybe, maybe not. What it does tell me is a trend. For a major trauma the protocol is to take vitals every 5 minutes and every 15 for lessor injuries. You need to catch and stabilize shock!!!
Which BTW is the only cause of death! These monitors will assist you in that.
For normal readings I use a sphygmomanometer(cuff) and stethoscope. It is actually pretty to use on yourself with a little practice. A caution is that you may hear a rebound and it will cause one to interrupt the reading higher than it actually is.
Another caution, like has been mentioned, is resting BP is best. However, when comparing BPs it is more important to have duplicate environmental conditions. Same time, same arm--it will vary from arm to arm--same pulse rate, same time before or after eating, same body position. Other wise while you may be close you will still be comparing apples to oranges.