Have used the Sigma 9 for over a year now. Excellent little HRM for the money. (Think I paid £29.99 at CRC for the old cream coloured one with a square face.)

No obvious problems. Would recomend the Sigma but if your budget allows go for a Garmin Forerunner 50. Its got some really trick software that allows you to link it to your PC. It produces slick little graphs of your performance etc. My wife bought one last year for £30 from Amazon. Must have been a bargain as I have never been able to see them again for anything less than £90.