My Galaxy Tab lasts a good 2-3 days before needing to be charged. I use it for about 20 minutes at a time at least 5-6 times a day, although mine is flashed with the overcome rom which I'm told improves the battery quite drastically. I use it for almost everything now, email, web browsing, call making, text sending, video playing, game playing, music listening(winamp widget ), GPS navigating and even comic reading(reading The Walking Dead on the Tab while taking a poop = heaven).
When I was considering buying it, I had the same questions going through my head. I already have a laptop and a phone so what's the point in a tablet? And to be totally honest, ever since I bought it I've used my laptop all of about 3 times in a month, and that was only to capture video. And my phone? Ugh, hate it now. I'd rather use the tab as a phone than my crappy Nokia.
If you're planning on getting an SGS2 then don't bother with the tablet. Aside from the screen size the SGS2 will kick mud in the Tab's face all day long. Once my SGS2 arrives I foresee the tab doing nothing more than collecting dust.