Picked up an el-cheapo Kenko Pro 300 1.4X teleconverter the other day. Well, not so much picked up as received via Ebay from some vendor in Hong Kong. :) For as inexpensive as it is, most tests show that this teleconverter is equal to the $300+ Canon teleconverter in terms of image quality, though it's not weathersealed. (Not a big deal -- my 20D isn't weathersealed, so weathersealing is wasted on me, anyway.)
In keeping with tradition, I went down to the back-alley parking lot of my apartment and shot some pictures of a squirrel munching away at the feed that our superintendent likes to set out. This was tested with a Canon 70-200 2.8L IS and the 1.4X TC @ F4. (Basically, wide open, but the TC reduces the available light to F4) On a 20D, that's effectively about 450mm... quite a bit of reach at F4 for a lens that only weighs a few pounds! Click to enlarge:
The above is cropped in slightly from the original.
This is a 100% crop of the image to give an idea of the detail. It has been unsharp masked, so this isn't raw lens performance, but it gives you an idea what's possible with this lens and the 1.4X TC.