I have had a vertical grip on every camera I've owned since around 1994. They've become a necessary part of my cameras, and it's ALWAYS the first accessory I purchase with a new body.
A lot has already been mentioned, so I'll just gloss over the normal talking points:
It's a sturdy grip for me. I don't have any flex issues whatsoever. Having two batteries in the grip allows over 1000 shots before a recharge is necessary. When I go on trips, I always bring a battery charger with me, but I have literally NEVER needed to take it out of the suitcase, no matter how much shooting I do.
Additionally, the grip includes a AA battery tray that will allow you to use AA batteries in lieu of the BP-511's. However, this is an "Emergency Use Only" type of accessory, as your battery life will be significantly reduced. I carry it in my bag in the VERY unlikely event that I manage to drain both of my BP-511's. You can find AA's anywhere in a pinch.
The grip is a real benefit to those like me that have big hands, since all of your fingers can now rest against the camera instead of curling up underneath. It's also a great counterweight when shooting with heavy lenses such as the EF 70-200mm f/2.8L. The arrangement of the vertical shutter release and related buttons on the side makes for very comfortable vertical shooting.
Okay, so it's a 5-star grip! Here's the only reason I took off one star:
The Canon EOS 40D and 50D have another button behind the shutter release, in addition to the two that are there on the 20D and 30D. The AF-ON button is located to the left of the '*' button. Those of us that do a lot of high speed continuous shooting often like to use the AF-ON button to activate autofocus instead of the shutter-half-press method.
Unfortunately, this grip does not include an AF-ON button on the vertical side. It only has the '*' and the AF point select buttons. So as it stands, if you have the AF-ON button set to activate autofocus, there's no way to autofocus when you're holding the camera vertically, unless you ignore the vertical grip buttons and reach your hand over as if the grip was never there.
There is a workaround, where you can set '*' to be the AF activation button. Since the grip has the '*' button on the vertical side, no problems. But then when you're reviewing images on the LCD, pressing '*' (normally zoom out, a feature you probably use all the time) will actually blank out the LCD and force you to re-enter review mode. AF-ON takes the zoom out functionality now, but it's really hard to retrain someone who has used '*' to zoom out on every DSLR they've owned since 2003. And I could be wrong, but I'm pretty sure that FEL functionality is gone once you swap the buttons in this manner. It's CERTAINLY gone when shooting vertically. Not sure about horizontally though.
Again, note that this problem ONLY applies to the 40D and 50D. Since the 20D and 30D do not have an AF-ON button, this is a non-issue for owners of those cameras.
Long story short, the grip needs an AF-ON button badly. But everything else about it is great.
