eBay score - Vivitar 2x Macro Focusing Teleconverter P/K-A R-P/K

I started out searching for a set of extension tubes to add some macro capability to my newly purchased Pentax K-x. After the requisite Googling I found not many of the tubes would pass through the aperture information which is a feature I'd like, and the only ones I could find were not no longer made and not available. In my search I came across some forum posts talking about this Vivitar TC and it seemed like it does just about everything I want. It magnafies your image 2x, so the SMC A 50mm I use with it becomes a 100mm. It also goes down to 1:1 which is considered "true" macro (meaning the image cast on the sensor is actual size). There were a couple on eBay and I scored one for about $75. For a macro hobbyist it's great.

Here's a flower:

Some morning frost on the car window:

These adapters are also no longer made, so eBay is your friend (or maybe KEH). Also be sure to get the one that says "P/K-A R-P/K" to be able to pass aperture info between the lens and camera.