The general rule of thumb is 1/focal length for handholding. So, if you have a FF body, you'll want 1/200 s at the long end, or 1/320 s on APS-C, to avoid blur due to camera shake. That means lots of light (outdoors), a high ISO (noise), or a tripod. IS counteracts camera shake. Also, the IS version is a bit sharper than the non-IS. Finally, if your body has some weather sealing (e.g. 7D), the IS version is a sealed lens, whereas the non-IS is not.
If the IS fits your budget, I'd recommend that over the non-IS. If not, the non-IS is an excellent lens.