I put an Optima in my Porsche a couple years back. It was a definite improvement. When the 12v battery in my Prius bites the dust (hopefully 3-4+ years from now!), I'll likely pony up for an Optima.
I replaced the OEM when it died last fall with an Optima from elearnaid 12 Volt (12v) Toyota Prius Auxilary Battery for 2004, 2005, 2006, 2007, 2008 and 2009 with installation kit and free shipping. Now $138 through Aug 31 (including $25 rebate). There is some work to installing it since it has 'normal' sized posts rather than the smaller ones on the OEM. But the instructions are good. It should be a better (more Amp Hours), longer lasting battery than the OEM.