The Canon BG-E2 Battery Grip adds the Canon EOS 30D and EOS 20D's missing part of the hand grip. It gives the pinky a home. It gives you more control over your larger lenses. It gives you a vertical/portrait grip with its associated controls and shutter release. And it gives you room for two Canon BP-511/511A/512/514 Lithium-ion battery packs.
The BG-E2 installs into the 30D/20D's battery compartment. The original battery door is easily removable and the BG-E2 plugs right in. The attachment is completed by screwing the BG-E2 into the tripod adaptor threads. The BG-E2 provides its own tripod adapter threads - the functionality is not lost.
The downside to the BG-E2 (other than having to purchase it) is the extra size and weight it adds to the camera body.
Want some added convenience when traveling? Need to get through an extra long shoot but do not want to buy the extra batteries required for it? Add the BMG-E2 AA Battery Holder for the Canon BG-E2 Battery Grip to power your 30D/20D with 6 AA batteries.
This is a nice accessory for 30D/20D owners to have.