I've been using ithlete for almost 18 months to determine if I am ready to exercise that day or, if it should be a rest day. It does this by measuring the variability of the r-r interval of the heart beat and then calculating a value that it compares with previous readings. For me, it quickly detects impending illness such as a cold and, warns if I haven't recovered from the last exercise session. The downside is that it won't provide readings if it detects an irregular heart beat. It also seems to be influenced by the dose of prednisolone.
I'm still undecided as to whether it's helpful to those of us who exercise with AF, but I'd be interested to hear your experiences.