Hear is
a link to the first big trial I did using the phone for caching, which describes the battery life. I have not noticed the battery worsen in the following months, but I have not found that many caches in one hit since either.
You might look at the battery life that I described there and decide it is poor. But compared to my GPSr it is good. With a set of Duracells in my GPSr they would of been used up almost completely by the end of that day. I have to say though that my GPSr is not a modern one.