Hi Stuart, happy to read another GS 300h owner , I have same car, totally satisfied with it. Don't worry about battery life. If you have a specific question ask, if you like to read more about GS, you can read in forum topics regarding the little sister IS 300h with same engine. And if you like to read tests, here is one:
http://carproductstested.com/cars/2014-lexus-gs-300h-premier-review/