Summary: I just (as of 1week) bought a Pentax K20D (and waited patiently for over 6 months to afford it!), and aside from a stellar opinion of the camera, I do suggest getting an extra battery. Having said that the pentax battery lasts very long (5days without charging) and will last longer if you turn off the monitor and/or lower it's brightness.
Summary: If you are serious about your shooting then you already know that the extra battery is a must. Nothing is more tragic in any photography excursion than to run out of power at that crucial moment when the light, subject and your vision come together magically. That is where this extra dynamo of energy comes into play. Never loose that magical shot due to power loss, carry the extra battery.
Pros: Easy to Install, Fast Recharge, Long-Lasting, Reliable, Widely Compatible
Summary: Same as the one that came in the camera, but essential to have an extra. Hiking through Bryce Canyon I ran out of juice, so having an extra charged battery kept me from cussing and kept me shooting.
Summary: I use the product for cases where the regular battery that comes with camera is flat and I need immediate replacement. I am a photo enthsiastic and I dare to work as a semi-professional in events organized by my friends. The product fits perfect to my needs and it's been working nice so far.
Pros: Charges Quickly, Good Value, Long-Lasting, Reliable
Summary: Any digital camera uses a lot of power, so I always have at least three extra batteries. With four batteries and a couple of chargers, one can shoot all day without having to worry. Running out of power just as the lighting you have been waiting all day for is about as frustrating as it gets. I wish they were cheaper, but they do work and last a couple of hours.
Pros: Charges Quickly, easy load and unload, Good Value, Long-Lasting, Reliable