I do quite a lot of close up photography, and needed extension tubes through which autofocus and TTL metering worked. The Canon tubes at 12mm and 25 mm worked out at just under £200 for the two of them. On looking round I saw the Polaroid tubes were well reviewed, with one proviso, that they seemed quite tight fitting. I purchased a set of the Polaroid tubes, 13, 21, and 31 mm, and find them to be beautifully engineered, with no problem of fitting, either singly or in combination with each other.
I take many photos stopped down to F32, and have used both natural light with long exposures and flash, and exposures on the AV setting have been fine. I have used both Manual and Auto focus on the lens with no problems either.
Although the tubes are made of plastic, the metal end-plates fitted will I am sure, protect the rings from excess wear, and they seem very robust.
As far as I'm concerned these tubes seem to be a very good buy and exceptionally well made at a very reasonable price.