Conventional theory is the higher end video and he video you want to use the higher class is needed. I think I have a class 4 I got somewhere along the line that has no problem rocking my HD videos in full 1080. I do not take 10 minutes worth of videos or things like that, but I do roll to 5 minutes frequently. Play back is also not hindered.
So I guess what I am saying here is the more speed you want the more power you are going to need and that would lend itself toward a higher class card.
Second idea.... if you can afford that sucker.... buy it and have some fun knowing your media is smoking fast in loading and saving.... just cause you're cool!
If you plan on doing some 1080p I would suggest going to at least a Class 4. Beyond that, I doubt you'll notice much difference. It shouldn't make the "apps" themselves run any faster though (i.e., the lag time to boot camera, etc.)
Full disclosure: I have Class 10, which runs fine, but only bought that over a Clas 4 because I got a deal on it (and it was cheaper than the same-sized Class 4).