Battery grips for dSLRs - are they worth it?  Having used them for years, I say yes.  There are - at least - four things they can do for you:

1) Allow you more battery capacity - as they take two batteries at a time.  However, this rarely is an issue - since typical-size cards run out long before the battery.

2) Allow you to use ordinary batteries.  They typically come with an adapter for “AA” batteries - or it can be bought inexpensively; this can be essential if your story is in some area without power.  Sure, some chargers will run off your car’s cigarette lighter - but charging that battery pack will take an hour.

3) Balance.  They really do help a lot balancing a long or heavy lens - and in keeping it still.  It’s a lot easier to hold a 70-200 f/2.8 “L” IS USM steady when the dSLR has a battery grip on it.

4) Vertical photos.  Most - if not all - battery grips have a second shutter release positioned to make shooting vertical-format photos easy.  They also make shooting vertical photos steady - unlike the camera’s own shutter release.

   My recommendation - based on experience - is to buy one that the camera manufacturer sells.  Sure, it costs more - but they are available used in excellent condition from the used division of Adorama.