I personally shoot in vertical/portrait orientation as much as in horizontal/landscape orientation. Having the vertical grip allows me to shoot from a much more comfortable and much more stable position - and gives me more control over larger lenses. The BG-E2N is invaluable for long sessions of vertical use.
With room for two Canon BP-511/511A/512/514 Lithium-ion battery packs, the BG-E2N gives us twice as many shots without a battery change. It will function fine with one battery as well.
The Canon BG-E2N Battery Grip installs into the 40D/30D/20D's battery compartment. The original battery door is easily removable and the BG-E2N plugs in. The attachment is completed by screwing the BG-E2N into the camera's tripod adaptor threads. The BG-E2N provides its own tripod adapter threads - this functionality is not lost.
The battery grip provides controls and shutter release identical to the camera's grip. The one exception is that the "AF-ON" button from the new-at-the-same-time Canon EOS 40D is missing. The BG-E2N replaces the Canon Battery Grip BG-E2. It basically adds some weather sealing around the battery compartment to the original design.
The only BG-E2N downside I can think of (other than having to purchase it) is the extra size and weight it adds to the camera body. The BG-E2N measures 5.6 x 4.4 x 3.0" (142 x 112 x 76mm). It adds 2" (51mm) to the height of the digital camera when attached. It weighs 10.2 oz (290g) without batteries.
Want some added convenience when traveling? Simlpy remove the grip and put the battery door back on. Need to get through an extra long shoot but do not want to buy the extra Lithium Ion batteries required for it? Insert the included BGM-E2 AA Battery Holder for the Canon BG-E2N Battery Grip and power your 40D/30D/20D with 6 AA batteries.
If you have a 40D, 30D or 20D, this is an accessory worth having.