I used to have the Canon EF 75-300 f/4.0-5.6 III. That is the cheapest 75-300 Lens from Canon. It was quite ok and i made nice pictures with it. Some of them though turned out blury so i decided to give it away and then i bought the Canon EF 75-300 f/4.0-5.6 IS USM. Since then i get sharper pictures. Yes on the upper end of the Zoom it does get a little soften but so did the other one. The only thing you have to keep in mind is that using the IS takes about one second before it works. Also the autofocus is a little bit slower with the 75-300 lens. Not slow with my Canon EF 28-135 f/3.5-5.6 IS USM. And yes of course... You can switch the image stabilzer off when you do not need it. When using a tripod you should definetly turn the IS off.
IS works best when taking pictures of static objects. It works ok when panning moving objects but you have to practice and find out when it suits best.
Just to give you an idea.. The photo below i made with the EF 75-300 IS lens. I scanned it from a print not a slide.
Click for large version
Photo © Edwin Olinowetz
Oh.. I almost forgot! I decided for the 75-300 and not for the 100-300 so i am more flexibel and do not have to change lenses so often. "L" lenses would be the best but they cost a looooot of money hehe..
Hope i was of some help.
Lot´s of luck with your new lens whatever it will be.