I have been looking for this exact product for a while. Sure I could have cut apart my power adapter, but I like being able to plug it in to stuff. Besides, that would have meant dealing with stranded wire which works great the first one or two times.
My only gripe is the + / – like you said. Labeling them center/ring would have been much more accurate. Regardless, this item is perfect for supplying power to a breadboard and I highly recommend it.