Picked up an extended 3600mah battery and cover from amazon.
It has been GREAT!
Can actually use the phone w/o micro-managing battery usage. Used Golflogix for 18 holes! And STILL had a usable phone afterwards. Even forgot to turn off gps for a couple of hours after the round, but still got through the day with a usable phone.
It really makes a huge difference in usability.
Disadvantages: the phone becomes a tad bulkier. My really slick Seidio Surface clip no longer fits - though I am planning to do some cutting and MAKE it fit. The Nite-Ize sideways case LARGE works well. Medium - not so much. And the Samsung mobile bracket probably would not fit (I didn't get one).