What is your opinion on vertical grips?

I'm a big fan of sling straps.  I naturally thought that by extension I would be equally enamored of sling bags, but after having owned a couple I've never been satisfied with the unequal distribution of weight.  The perfect camera bag is kind of like the photographer's holy grail... much doubt such a thing even exists.

If you're shooting a lot of portrait shots, don't mind the extra weight/bulk, and/or need the extra battery life, then you will find a vertical grip to be a valuable addition to your gear bag of choice.  I've shot with them for years, particularly enjoyed using them at weddings.  Yes it does do a great job of balancing the weight of the gear, but at the expense of adding weight...you have to decide what's more important to you.  After carrying one or another for years I downsized; I don't miss the extra weight, but then again I don't do that kind of photography much anymore where a grip would really be a benefit.  It's an effective tool for your camera, if you have a use for it.