Helo formation
I've flown a ton of formation, pretty much standard operations for my line of work. Basically, we fly two types, close formation less than 3 disks, and tactical, greater than 3. Note that tactical formation, while not being "close" is still considered formation because there is a definite formation lead and you utilize tactics that maximize mutual support and defensive maneuvers.
I'm sure that by the initial post we are talking about close formation here. It's quite fun to fly, even at night on NVGs, you get pretty comfortable with it. The key for flight lead is to remain predictable, maintaining a stable platform and gentle maneuvers. This gains more significance as the formation gets larger. I must admit that while it's fun, the only tactical use for it is mass on target. It's important to insure that the ground forces get as many troops on the ground in the shortest amount of time. That said, about the only time you get this close is during intial takeoff and during the landing phase, everything else is usually spaced out quite a bit.
Another use for formation is when air refueling, that too can be fun and quite safe. I tend to fly some close form in training simply to keep my skills up. As far as safety goes, don't quote me but I'd bet that there are plenty more midairs between non-form aircraft than formation aircraft.