One of the biggest drawbacks of the iPhone 3G, even after firmware 2.1, is its poor battery life. For people that tax its battery with extended use of the 3G, WiFi and Bluetooth radios an external battery is practically a requirement. On a recent trip when I was using my iPhone 3G a lot I would get the 20% battery warning before noon every day.
The new v2 release has newer software built into the battery which allows for tandem charging with the 3G iPhone and the RichardSolo battery when connected to the AC wall charger. The previous release also had this feature, but in some instances was not charging the 3g iPhone when both were connected together, and to the wall. According to the manufacturer the v2 battery version has better manufacturing techniques and is less likely to experience problems.
The external battery works with all models of iPhone and iPod, including the 3G, except for iPod shuffle and the kit comes with the 1200mAh battery, retractable USB charging cable and a 110-240V wall adapter.