Today I finally received the lens for which I sold the 300mm f/4 IS (and a couple of other lenses), the 300mm f/2.8 IS along with a 2x tele-converter. I intend to use this lens bare and with the TCs, but I will admit the prospect of having a 600mm f/5.6 IS (with the 2x TC) that is hand-holdable was a major factor. I will admit though it isn't entirely a 'poor man's 600mm' but it sure is a lot cheaper than the Canon 600mm f/4 IS (which costs a smidge over $6500). I'm looking forward to doing some bird and wildlife shooting with the new set up in the coming weeks.
I went into the backyard hoping for some robins but unfortunately none were to be found. The moon however was out, so I grabbed the IR camera, mounted the 2x TC and lens, turned on IS and snapped this pic. Not too shabby overall considering it was done hand-held and with a 2x TC (which is notorious for degrading image quality).
Technical Specs: Canon Rebel XT IR, 300mm f/2.8 IS + 2x TC (600mm equiv.), f/6.3, 1/400s, ISO 800.