Those of you who have one of these phones would agree with me. By far, this is the phone with the longest battery life of all the new Windows Mobile 6 devices we've seen. I haven't stress tested it yet but my previous phone battery lasts around 8-9 hours before you need to charge it, with 3G and DirectPush enabled the whole day.
For the Blackjack II, I have three bars left come midnight with everything turned on and regularly reading and downloading RSS feeds.
Go get one now : http://www.wireless.att.com/cell-phone-service/cell-phone-details/?device=BlackJack(TM)+II+by+Samsung&q_sku=sku1140027