Granted, I don't have the book in front of me, but doesn't Toughness increase to 10 at Paragon, and 15 at Epic? Granted, that won't make any difference at Heroic, but taking that into account, it doesn't seem terribly powerful to me. A good choice, but not THE choice, you know?By the way -- if all your players have the same background, try and find a way to tie it in to a plot/story.
How is it that they are all born under a bad sign... perhaps cosmic forces brought them together for a reason, etc.
As DracoSuave said, sure, it won't break the game since we're talking roughly equivalent to slightly more than a toughness feat. But at the same time, if someone chooses a skill-benefit background, they get less than skill focus would give them (also a feat) so getting a background benefit that is better than toughness (also a feat) is slightly (although not detremintally so) stronger than the other choices.
I personally just find it plain boring though.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.