I've had a Transcend card before which died on me, and I heard they tend to do that often, so now I only use the Sandisk Extreme cards.
In a 550D I tried both class 6 and class 10 cards, and for taking photos there is very little difference.
When taking videos, if you're using a class 10 card you can take pictures "simultaneously". The video will freeze for 1-2 secs, but will resume after the picture is done taking. With a class 6 card, after 1-2 pictures the video would choke up and die. With the class 10 card, the video can handle you taking multiple pictures with no issues.
Another thing to consider is the transfer speed to your computer. Transfering a lot of files takes a lot of time. I think a 16gb class 10 30mb/sec card takes something like 8 minutes to copy, something like that. It depends how much of a hury you are into. I'm not a professional, so I don't care, but if I transfered 1000s of pictures a week, I'd start to care more.
The other important factor is durability. How often do they become unusable, and how well do they resist weird things such as falling in water, etc.
Cameras: Canon EOS 6D, Canon EOS Rebel T3i, Canon EOS Rebel T2i, Canon S90
Lenses: Tamron: SP 24-70mm F/2.8 Di VC USD, SP 70-300mm F/4-5.6 Di VC USD, Rokinon: 8mm Fisheye cine, Canon: EF-S 18-55mm f/3.5-5.6 IS Lens, 75-300mm f/4.0-5.6 III, and EF 50mm f/1.8 II
Retired camera: Fujifilm Finepix s700