I used the ithlete every day when I was training hard last year. Like Henry says, it is pretty reliable. It always told me when I had a virus coming up, about a day or so before I felt any symptoms. I didn't always do what it told me to do (ie. I sometimes raced on amber or red days), but at least I understood why it didn't quite go well or I felt less comfortable running. There were also days when didn't think I felt good and, had it not been for the 'green' reading on ithlete, I might have canned the session. Generally, these sessions ended up being good ones.
I'm not really running at the minute due to a long-term injury and have put it on occasionally out of interest, to see what my reading is. It's a bit depressing as I now score 20 or more lower than I did this time last year, showing how unfit I've become! When I get back to training, I will definitely start using it properly again.