I have a set of both SAE and metric, but I have to admit that I don't use any of them too often. On the very rare occasion that I have needed one, it seems like I'm usually using an SAE size. To be honest, I don't recall ever using any of the metric sizes. Coincidentally, I used a crow's foot to install a kitchen sink about one month ago. Prior to that, I don't recall the last time I used one. I've had both sets for at least 15 - 20 years and have used them no more than a dozen times at the most.
Looking back, I might not buy them again unless I had a specific need for one. But as a long time subscriber to the "When you need it, you need it." theory of tool acquisition, I guess a set of crow's foot wrenches is nice to have on hand. Let's face it, no one likes to stop in the middle of a job to go buy a necessary tool. That being said, I'd still put them low on the list of "must have" tools. I'm not so sure they really earn their keep.......but when you REALLY need one, it's nice to go to your box and pull one out..... An inexpensive set may be the way to go because you might not get your money's worth out of them.
Jim C.