As the saying goes, "Get a grip!" And if you happen have a Canon EOS 5D Digital SLR, get a Canon BG-E4 Battery Grip.
One of the significant differentiators between the Canon EOS 5D Digital SLR and Canon's 1-Series bodies is the lack of a built-in portrait grip. Add the BG-E4 and this difference is gone. But so is the size and weight difference (the BG-E4 weighs 1.375 lb/.17 kg with batteries). But, the BG-E4 can be added or removed as desired - giving you the best of both worlds.
The battery cover on the Canon EOS 5D Digital SLR is easily removable. See the small slider on the inside portion of the door hinge? Pop the door off, slide the BG-E4 into the battery slot and tighten the BG-E4 into the tripod insert on the camera body.
Two inserted-from-the-back battery options are provided with the BG-E4. My recommendation is to use two Canon BP-511A Lithium ION batteries allowing about twice as many shots without a battery change. The other BG-E4 battery option uses six easy-to obtain AA-size batteries (alkaline, lithium or Oxyride) in the supplied BGM-E2 battery magazine.
The Canon BG-E4 Battery Grip costs significantly more than its Canon battery grip predecessors, but it is more ruggedly built. Like the EOS 5D, the BG-E4 body construction has magnesium alloy incorporated into it for strength. The only attribute I don't like about the BG-E4 is the too-square grip. Although others didn't seem to mind it (or really liked it), I found the too-squared edges to be slightly uncomfortable. But not as uncomfortable as holding the 5D in portrait orientation without the grip installed.
The surface of the full-size portrait grip is rubberized for sure handling. With the BG-E4 installed, larger lenses are much easier to control and portrait oriented shot are more comfortable to take. If you own a Canon EOS 5D Digital SLR, consider adding the Canon BG-E4 Battery Grip to your kit.