I can get 6+ hours out of a single charge with the 6 cell battery, so to me, it's worth the ruined aesthetics. Also, as others have mentioned, it lifts up the back of the unit just enough to let in a little extra air for cooling and also to angle the keyboard comfortably for typing.
I also ordered the neoprene sleeve (I think by Belkin?) that Dell sells and it does stretch to accommodate the larger battery. It's not the best quality sleeve, but it works for my purposes. If you think you'll be rough on it, though, you might be better off saving the money and putting it towards something a bit more durable.