I considered it, but you also need a bigger back cover, and I didn't want that. Instead I bought a set with two extra regular batteries and a wall charger on eBay for only $9.99. I also use the free version of JuiceDefender on aggressive which helps.
Here's a video about an extended battery. It shows the Optimus V, but the description mentions it's also available for the Slider.
Extended Battery 3600 mAh LG Optimus V / U / S - YouTube