My everyday zoom is the most excellent Canon EF-S 55-250 IS, which is ultra sharp and has that ever so useful IS for hand holding at lower shutter speeds. It is however, an EF-S lens, which means that it is incompatible with most 2x Teleconverters like the Jessops one I picked up from the car-boot sale last year for a fiver.
The old push-pull 100-300 is an EF lens, so it is compatible with my Jessops teleconverter, so with it I can push out to 600mm, or if you take into account the crop factor on my 350d, an amazing 900mm.
There are of course, compromises, and with the EF 100-300 f/5.6 they lay mainly around its' slow f/5.6 maximum (although constant) aperture, which keeps shutter speed down in lower light situations, which can lead to blurred photographs.
The answer could be to put your camera on a tripod, which is fine for static subjects but no good for moving ones, or if you've got a newer camera, wack that ISO up to get the shutter speed fast enough to eliminate camera shake and subject blur. The most practical solution might be to recognise how to get the best out of the lens, stop it down to f/8, and then use a tripod for fixed subjects, or only shoot moving ones on Sunny days!
Using an extender poses more problems with light. A 2x Extender cuts out two stops of light, which means it turns the lens into the equivalent of f/11, or f/16 if we've stopped it down to increase sharpness. To put it another way, if the shutter speed was at 1/500th without the extender, it'll be 1/125th with the extender, enough of a reduction to introduce an awful lot of camera shake and subject blur. The final problem with using an extender on this lens is that you loose auto-focus, so it's back to squinting through that view-finder and twisting the barrel in the old-fashioned way.
Don't let this put you off though, because as you can see from the samples below, which are all hand-held, you can get good results, and I'm sure with practice I'll get even better images.