I got a SigmaSport HRM (don't recall which model - it was ~ $40) from NashBar last April. It worked fine for a month, and then crapped out despite replacing battery. I sent it back and got a replacement which never worked at all -- period. Sent it back again, and this one's been working fine.
I have a wired Astrale-8 computer, so interference had nothing to do with it. I emailed SigmaSport after the second one, thinking that I must have done something wrong or there was a secret setting, but they just said to send it back. I never did find out what the problem was, but you would think with a reject rate like this that it would be hard for them to make any money...