I fly a Super Cub (my dads) and have some time flying formation with it - and it is quite a bit easier to fly formation in real life than it is in the sim. You have more cues to let you know what the plane is doing and how you're moving in relation to the other airplane. You've got the sound / feel of the engine, feel of the G's, feel of the controls, and also have better depth perception so it seems that you can catch relative movement a bit quicker than you might be able to in the game. The controls are much more solid feeling than a joystick, there isn't that annoying clunk near the center when traversing X and Y axis - so it allows you to make smoother and finer control inputs. The tendancy to porpoise is greatly reduced as well in the real airplane. It seems that the pilot induced oscillations (PIO) are the biggest problem for people flying tight formation in the sim.
One reason that it's probably easier in real life, in my case, is because I'm not going 300+ MPH,

The faster you go, the control inputs required to move a certain distance in a certain amount of time become smaller, so it's probably fairly easy to get PIO flying in a faster/more maneuverable airplane if you're not familiar with how it's going to react to control inputs. However, there's also much more stick force required to pull X amount of G's in most high performance airplanes, so that helps reduce the tendancy to PIO. I think I read that it's 20lbs of stick force per 1 G in the Mustang.
With practice though, you can get pretty good in the sim. I've always really enjoyed flying formation and like to think that I've become fairly proficient with it in the sim - the concentration required to fly it well really makes the time go by quickly while I'm enroute to the target.
Here's a video I made a while back of some AHII formation flying in '51's - I was flying too deep most of the time.
http://www.youtube.com/watch?v=Qd4O3UIB0gw&feature=channel_page and here's some Cub formation - you can see how it doesn't look nearly as twitchy as in the sim (but it's also not a high performance airplane, lol)
http://www.youtube.com/watch?v=7P8y2V00X-4&feature=channelWolfala has some cool videos of him in a Cirrus SR-22, and even though it's a faster airplane, you can tell it's still very stable and not twitchy like the airplanes in the sim - which is a result of the limitations of most joysticks.