Ya, I've got an extra battery for mine. It's well worth the extra $90. I would defnitely recommend it if you use your laptop on battery for longer than 3:30 hours in one sitting.
As far as battery life with undervolting, mine's at 0.700 for 6x, and the longest I've stretched it to is 4:30 hours. However, that's with the screen on the second most dim setting, video card underclocked, wireless and bluetooth off, and only typing in Word for the entire time (it was a paper that was due the next day lol). Under normal use, I can get about 3:00 to 3:30 hours of battery life. This is with wireless on, screen at about 3/4 to full brightness, video card at full clock, and possibly playing music with iTunes.