I got it when it first came out, and I can't go back. I think it's equal parts the extra grip it adds to the base as it is the extra battery life. And it's not actually double battery life, it's a tad shy of that.
But mine is still holding up well, can't recommend one enough; and especially for $20, it's the way to go.
http://www.backloggery.com/darrentg R.I.P Ronnie James Dio
I got it a few weeks after the system came out and I totally agree. Im never going back. It's like DSi battery length and its nicely design. The grip is nice but it does make it alittle thicker making it even more harder to store in a pocket.