There are plenty of different extension tubes available on the market, a lot of them considerably cheaper than these. One of the reasons these are a bit more expensive is that they can send metering information through the lens to the camera, though that’s not of great importance since you’ll probably be in all manual when you’re working that close anyways. It’s a relatively cheap piece of equipment and it does what it’s supposed to.
One thing worth noting is that you probably don’t want to put a really heavy lens at the end of the full kit, unless you plan to support it with your second hand. The way I shoot macro means I often have a bug in my other hand and can’t always brace the lens on anything. In those cases the tubes kind of bend a bit. Just something to keep in mind if you happen to use a lens like the 105mm f2.8.