iPhone 4's battery life has been real good for me. I don't see a need of extended battery for weekdays, but these might come handy for weekend camping trip or something. I've never heard of this Exogear company, but the design looks OK and thinner than those I've seen for 3GS from Mophie and incase.
http://www.exogear.com/products-iphone-4.html
http://www.exogear.com/products-iphone-4.html