I've got a GSG-5, and it sees a lot of range time, but it can be finicky. It's cool, and everybody who's shot it likes it, but it does still seem more of a novelty, albeit an accurate shooting one.
The Sig's got the advantage of a larger capacity mag right of the bat, 30, compared to the GSG's 22 rounds.(can squeeze 24 in there though) Plus the mags look to be Stanag sized, so they'd fit in most pouches, assuming they don't have some weird protrusion that sticks out farther than a normal mag. Not to mention it's priced competitively with the GSG-5 at $515(msrp).
Downside, it has the little red dot, but no BUIS rear, although one is available separately, and it looks like any 1913 standard one should work (like Magpul's new one)
I'll be keeping an eye on this one.