It can be completely accurate, I had my wrist BP monitor checked at my local GP's surgery and my GP said that my monitor was actually more accurate that their upper arm one, ask at your surgery if they will check yours for you.
Yes Illyria is right - i took my small, wrist BP monitor (cost �30 off the net) to work and took a reading, and then asked one of my nursing colleagues at work to take it with a 'proper' monitor which they use on the ward - the reading was exactly the same.