IMHO, the basic concept is good, but there are problems in the execution. First, I've not seen any other calibers offered, only 9mm. Second, Sigs are expensive to begin with the added calibers will cost as much as some other guns. You could easily have over a thousand dollars into a gun and conversion kit.
The gun itself is a nice little DAO only shooter. Some don't like the lonnnnng takeup but that is because it is a true DAO....not a partly cocked model. The DA is rather light, smooth and breaks clean.
Pros: Nice trigger. Double strike capability. Comfortable.
Cons: Expensive. Still waiting for other calibers. Caliber conversion kits are expensive. Some don't like the long trigger pull.
One neat thing about the gun, is the security. You can quickly remove the actual gun's firing unit, which completely disables the frame. This might be good if you have to leave the gun in the car or for traveling though I'm curious what the legal implications would be if you carried the firing unit into a place that was off limits to guns.
I've read a lot about handguns being stolen out of luggage and I wonder if this pistol could give you an advantage when flying. You could mail the non-gun parts to where you intended to be (no FFL required) and only carry the firing unit in your luggage. It would probably be less likely to be stolen because it wouldn't be a working gun...or even look like a gun.
Also, down the road I could see aftermarket companies coming out with unique frames and magazines to support the firing unit. Imagine a single stack 7 shot tiny frame.
