I don't see it affecting phone battery life at all -- it's a feedback circuit from the Sena mic to its speakers.
I haven't tested it on a long ride yet, so can't report on Sena battery life, but my guess is the hit will be negligible.
I wish Sena provided an adjustment for amount of sidetone -- it's a little more than I'd like at its present level, but that's nitpicking. In general it's a great addition.
2009 BMW F800 ST
Never let your motorcycle take you where your brain hasn't been five seconds earlier.