A measuring square is an essential tool for framers, but also quite useful for handymen. It is essentially a triangular piece of flat metal (or plastic) with a base on either side of one edge. Originally, one of its main uses was for assisting with laying out rafters, specifically, cut angles. Nowadays, it probably doesn't see too much use for that purpose, but it is still helpful when cutting angles. However, the uses don't stop there.
I primarily use my speed square for checking that corners are square, and as a cutting guide. It is definitely worth it to have one for those tasks. It can save a huge amount of time and needless frustration.
These are my two measuring squares. The one on the left is made by Swanson (the original Speed Square inventor) and the one next to it is a generic version. They both cost roughly the same amount of money - about $8-10. But there is quite a difference in quality and usefulness between the "original" and the "copycat."
First, the quality. They are both one piece of solid aluminum, but the Swanson is much beefier. The generic one is some sort of aluminum alloy. It doesn't feel like a strong metal. It kind of reminds me of cheap pewter jewelry. If I wanted to, I'm sure I could practically fold it in half. However, the Swanson feels much more solid. I can't help but notice the metal stamp of "SWANSON TOOL CO. INC. FRANKFORT, IL. 1987"
Now for the features. Well, they are nearly identical. I'm guessing the copyright ran out at some point and everyone jumped all over it. I mean, it's just a blatant copy, however, the Swanson does have some features which the generic does not. Take a look at the picture of the Swanson one above. See that diamond cutout near the top? That's to guide a pencil as you take the base edge and run it along a piece of wood. The cutout is at 3 1/2 inches, which is the actual width of a 2x4. This is useful for resawing a 2x6 or 2x8 into a 2x4, or for cutting out plywood to thicken a 2x4.
Those little nubbins on the cutout of the inner triangle are also pencil guides. They are useful for marking out widths from 1 to 2 1/4 inches in 1/4" intervals.
As a cutting guide, the Swanson really shines. In fact, this is what led me to buy the Swanson (the store that I was at when I needed one didn't have the Swanson). The generic version is about half as thick as the Swanson. This not only makes the Swanson more durable, but a much better circular saw guide. Circular saw baseplates are rounded up at the edges and don't usually have sharp edges. With the generic square, I've had my saw's edge ride up on top of the square as I'm trying to use it as a guide. This not only gives a poor cut, defeating the purpose of the guide, but it's dangerous! If it rides up too far you could be out a square, a blade, and even fingers. The Swanson's added thickness keeps the saw where it should be, beside the guide.
All things considered, I would definitely stick to the original and stand by the Swanson. Other generic versions may fare better than this one did, but at least you know what you're getting with the original.