The third party grips have a reputation of lasting from 3-6 months, depending on use. Just buy 10 of them and you'll be set for 3 years.
sources for such a bold claim?
i would love you write not such biased nonsense all the time about third party gear, you propably have never used yourself or any experience with.
i bought my 5D MK2 over 4 years ago and the BG grip i bought for it is still absolutely fine.
the phottix BG for my 550D is from 2010 and the 550D is my cheap "throw around" camera.
the BG is also still absolutely fine.
and im not a weekend shooter....
Actually, I was wanting to buy one for my 5D MK III, but after reading reviews on the various models, I decided to just skip them.
The issue is the 5D MK III grips. Its nice that your 5D Mark II grip worked fine, but there are tons of issues with the Grips for the 5D MK III, and I've read lots of reviews about them running down batteries, controls not working, locking up the camera, etc.
For example, read the Amazon Reviews. Things look rosy until you see what was actually written. The positive reviews often come from some clueless person who tells how good his 5D MK II product was and is not a review of the 5D MK III product at all.
Its like saying my 2005 Automobile was wonderful, so you will be happy with the newest model. http://www.amazon.com/Meike-Version-Vertical-Battery-Camera/dp/B0093Y0CXE/ref=sr_1_2?ie=UTF8&qid=1370627200&sr=8-2&keywords=5D+MK+III+grip http://www.dpreview.com/forums/thread/3205127