Accurate?... I've not found any yet that are inaccurate. However, they're not showing the cooking surface temperature because they're not on the cooking surface. On my brinkmann it usually runs 20 degrees hotter than the cooking surface. On my Lang it's about 30 degrees cooler than the cooking surface... This is because of where they're located.
After market, I've found that Tel-Tru work fine and are accurate.