I think there are three reasons why Surly is so popular.
1. They make a decent frame, with good features.
2. They listen to what bikers want. They jumped on the single speed/fixie popularity surge faster than most of the makers. The Big Dummy is a perfect example of that.
3. They aren't afraid to try different things. The Pugsley and the Big Dummy are examples of that. Who else has a superfat wheeled bike where the front wheel can be switched with the back in an emergency? The Big Dummy is certainly going to be the first production frame designed to be a dedicated Xtracycle rig.
I just hope they don't stick with the Big Dummy name though. If they're going to stick with the "Sanford and Son" theme, I would rather see them call it the Sandford or the Lamont, just not Big Dummy, despite the humor.
I really like their plain, single color looks too. Most of the modern factory bikes that try to go artsy, look horrible.