The way I've handled the group skill check is to simply assume that the better PCs are helping the less skilled PCs. So in the stealth example, if they are trying to sneak through the forest, you might have the ranger pointing out to the fighter and cleric where to step and warning them of potentially bad places to step. For an endurance check, the heartier PCs are keeping an eye on those who are more frail, or encumbered with armor, and showing them techniques to get through the chill or fight off the heat, etc. Sometimes though, there's only so much that you can do to help somebody out, so they still fail (represented as less than half the group making the check).
In my experience, I've seen a pretty decent mix of succesful and failed group skill checks. I don't use them real often, but when I do, its not automatic as to what the result will be. Now Aid Another on the other hand can quickly become a problem -- particularly when you hit paragon and the DC 10 is nearly automatic for almost all skills. Now that I am in Paragon tier I don't always leave the Aid Another DC at 10 for skill challenges, or I might limit the number of people who can aid, or better yet, force them to tell me exactly how they are aiding the wizard on the arcana check. Just because the fighter has a +7 to his arcana check at level 14 doesn't necessarily mean he knows how to assist the wizard with her +20 or so to the check.
In my experience, I've seen a pretty decent mix of succesful and failed group skill checks. I don't use them real often, but when I do, its not automatic as to what the result will be. Now Aid Another on the other hand can quickly become a problem -- particularly when you hit paragon and the DC 10 is nearly automatic for almost all skills. Now that I am in Paragon tier I don't always leave the Aid Another DC at 10 for skill challenges, or I might limit the number of people who can aid, or better yet, force them to tell me exactly how they are aiding the wizard on the arcana check. Just because the fighter has a +7 to his arcana check at level 14 doesn't necessarily mean he knows how to assist the wizard with her +20 or so to the check.