I think the SD was the updated version of the old Sigma starting around 2011 or 2012. The biggest complaint you hear about them is the trigger is a little on the heavy side, but that can be fixed to some extent with a kit from Apex.
These have always been a very solid and inexpensive gun. They always seem to function with anything you put through them and they generally shoot well. They tend to get frowned upon because they lack the "cool factor" and like I mentioned above, the triggers usually aren't very good, but if you are looking for an inexpensive gun that always goes bang and shoots straight, they're as good a choice as any.
When I bought my S&W 500, some years back, Smith & Wesson had a promotional deal going that with the purchase of a new 500 you could get one of their Sigma's in 9mm or 40 for $50, I gladly paid another 50 bucks and got one in 40. That Sigma may not have the cool factor of many of my other guns but I'd have zero concern grabbing it to protect my or my families life with it.