They are quite good. Constantly changing technology and designs have improve ease of use as well as acuracy of current glucometers. I would say around 5-10 points of actual blood levels when tested at a certified laboratory. They are meant to guide you with controlling your diet/food consumptions to maintain a good sugar level. Having diabetes is tough! Fllow-up with doc regularly. Good luck.
If your technique. Is correct and the strips are in date and stored properly -- you should be within 10-15% of the actual blood sugar almost all of the time. Get instructed by a good diabetes nurse educator for best performance.