I prefer the Narcoleptic library because it's so easy to use. You get most of the savings without any of the complicated code. Google it. Instead of using an interrupt pin to wake the chip, I wake every 5 seconds and digitalRead for any interesting conditions. This is much more flexible when there are multiple wake scenarios. The only disadvantage is how often I am checking. An interrupt pin effectively checks every millisec? And uses less power than my checking every 5 seconds.
Using the board from Yourduino, it only takes 10ma when sleeping, USB and 5v regulator.
Off the top of my head that's 10 days on AA batteries.
Pro mini only 1.5ma. Good enough or me!
It's about the same as the self-discharge rate of AA NiMh.
Much simpler than building your own. Much less than Uno 33ma, 3 days.
Watch out for Serial.print().
Depending on the baud rate you need about delay(2) before you go to sleep to finish sending a small buffer.
What's the largest buffer size in bytes?