Funny you should ask as I just disabled that feature today. I had it enabled and have been using it since release day, but have found the following "issues:"
If my finger is wet or frayed a bit from working outside, the sensor won't read it.
If I don't put my finger on the sensor precisely as I initially scanned it ( ie., maybe it is down 1/4" from where I initially scanned it ), it won't read it.
I ended up creating multiple fingerprints for each thumb to work around the issues and while it works most of the time, it failed enough that it is just easier to either enter a passcode or don't use that security at all.