I have a question about extention tubes. I want to do some more macro work but haven't saved up enough to get the 105 macro lens that I want. A craigslist came up recently for a Nikon AF Extension Tube Set -- Kenko

It includes all 3 sizes, 12,20 and 36. The price appears to be around 169. The poster is willing to take 100 bucks for all 3. I've seen some reviews where there was concern about the plastic construction but they seem to hold up.

Does anyone have experience with this product or am I better off looking for a superior set although more expensive.

Here is a product description: Kenko DG Teleplus Extension Tube Set

Extension tubes are designed to enable a lens to focus closer than its normal set minimum focusing distance. Getting closer has the effect of magnifying your subject (making it appear larger in the viewfinder and in your pictures). They are exceptionally useful for macro photography, enabling you to convert almost any lens into a macro lens at a fraction of the cost while maintaining its original optical quality.

The DG extension tubes have no optics. They are mounted in between the camera body and lens to create more distance between the lens and film plane. By moving the lens father away from the film or CCD sensor in the camera, the lens is forced to focus much closer than normal. The greater the length of the extension tube, the closer the lens can focus.

The KENKO DG AUTO EXTENSION TUBE SET contains three tubes of different length, a 12 mm, 20 mm, and 36 mm, which can be used individually or in any combination to obtain the desired magnification. Kenko also makes a DG UNITUBE 12 mm or 25 mm which can be purchased individually.

Kenko's Auto Focus extension tubes are designed with all the circuitry and mechanical coupling to maintain auto focus and TTL auto exposure with most Canon, Nikon, and Minolta lenses given there is enough light to activate the cameras AF system properly.

Please Note;

1. When using extension tubes the lens will not focus to infinity. The focus range will be greatly limited to a very close focusing distance.
2. There is light fall off when using any extension tube, sometimes the equivalent of 3 f-stops of light is lost when using multiple extension tubes together. This light lost can affect the camera's ability to auto focus. Manual focusing is recommended should the lens begin to "hunt" (not lock onto the subject).
3. Nikon "D" information is not transmitted because the lens is focusing closer than its programmed to focus.
4. Kenko Auto Extension Tubes are not auto focus compatible with Nikon AF-s (Silent Wave) lenses, manual focusing is required with these lenses."