Most are accurate. Automatic BP monitors in general are quite accurate, if you use the arm (not wrist or finger units which are fraught with potentially more conflicting factors) cuffs correctly. I prefer the ones that pump up automatically since your self-pumping them may raise your bp, defeating the purpose of home monitoring. Note that bps are quite inherently variable, so averaging several bps are better.
Consistency. The important thing is to use the same monitor consistently. The most important thing is the trends, hopefully going down and staying down, rather than the absolute number itself. You can also take it in to your doctor's office and calibrate it against the meters there.