I received an ad via email this morning for the Woodpecker 12" Square.
It looks like a fantastic tool: it's got a 12" blade and an 8" handle, the blade and part of the handle are machined from one block of aluminum, and the remainder of the handle consists of more machined aluminum slipped onto reference pins. It should stay square forever unless you beat on it with a hammer, and it's guaranteed to 0.001" accuracy.
Then I looked at the price. $99.99, for a square. yes, it's an accurate square, and yes, it's almost certainly well made, but still.... a hundred bucks for a square?
Let's look at an alternative, which happens to be the same brand as my 16" and 6" squares. It's a combination square, made by Empire Level. it has a 12" blade, although the equivalent of the Woodpecker's handle is only about 4" long. It, too, is guaranteed to 0.001" accuracy over its length. It costs about $15, or about 1/7th of what the Woodpecker costs.
Now: The Woodpecker tool has some advantages. First, the rule starts measuring at the edge of the board, not at the end of the blade, which means measuring with it is easier. Second, it's been designed to be a little easier to hold. Finally, it has a much larger registration surface, so it will be easier to keep it where it ought to be. All of those, particularly the last one, are valuable, and if cost were no object I'd probably buy one of each.
But...well, most of us have budget limitations. As I mentioned, I have one of the Empire squares in a 16" size. It's a little awkward at its full length, which probably extends about 13" from the board edge, but I've used it a number of times at that width with no real problem. It's accurate, it's repeatable, and it's cheap. For my money, it's easily a fair trade off. Here's a case -- one of the few cases, probably -- where you can easily go cheap, and not have to worry about giving up accuracy, quality, or longevity.