Under the Veritas Brand, Lee Valley sell five gauges in this style. Two square (small and large), two dovetails markers (1:8 and 1:6 ratio) and a 45 degree mitre gauge (which I will be picking up with my next LV order.)
A 'saddle square' is a simple laying out tool that is square (90 degrees) in two planes. This allows you to mark square across the face of a board, and without moving the square accurately mark down the edge of the board. This tool is super useful in hand tool work as it allows you mark the shoulders of cuts with precision. A very handy tool. A 'dovetail marker' is slightly different having a square side and a side at an angle for laying out dovetails.
They are made from anodised aluminium making them tough and light weight. A small notch in the inner angle is a nice attention to detail which helps to keep the surfaces flat and allow starting point for the marking knife. (They do however scratch easily with a marking knife if you get too close.)
Would I recommend them? Yep, I do. I use them on nearly every project. Of course you could make your own, or use a traditional square/sliding bevel. However, these gauges are relatively inexpensive, always set at the right angle, and are super easy to use.