Before I start writing about this lens I'd like you to remember one thing: a newer, better lens, the EFS 55-250mm f4-5.6 IS goes for about GBP£140 / US$180 on Amazon, with a years guarantee. Never, ever, pay close to that price for an older 100-300 or 75-300 non IS lens for a cropped sensor dSLR. Just save up a little more and buy the 55-250 instead.
Now we've go that out of the way we can concentrate on the push-pull 100-300 f/5.6, one of the first budget EF zooms, that if you can get for a bargain price is worth adding to your lens collection, especially if you want to experiment with cheap teleconverters to really reach in to the distance.
Instead of turning the lens barrel to zoom, you push it in and out. Once you've used it a couple of times this is a very natural way of doing things, but it also means that when a lens has had a fair amount of use they are very prone to zoom creep - when the camera is at an angle different to the horizontal the lens zooms out or in.
There's no Image Stabilization on this glass either, so really keep an eye on the shutter speed to avoid camera shake, or use a tripod. It is a constant aperture, but f/5.6 is pretty slow, and that also means that if you add a budget teleconverter, usually a 2x one, you'll lose auto-focus and you'll need to focus manually.
I got my example for £40, which I think is a fair price. I use it occasionally with a 2x teleconverter, but it by no means is a replacement for my Canon 55-250 IS, but if I didn't have that lens I'd be more than happy to try and get the best out of its longer, older EF counterpart.
In conclusion, if you haven't got a decent telephoto zoom and are on a tight budget, one of these push-pull lenses will do the job on bright sunny days, but treat it with respect and learn to get the best out of it.