IIRC you have to download the lens profile into Canons EOS Utility, check the documentation there. Then you can load it into the camera.
It will be applied only to in-camera jpgs. If you shoot Raw, you have to apply correction via the lens profile in you raw software ( Lightroom, DPP etc)
I do notice it when there are large expanses all one color (sky or snow for example) and there is noticeable fall off towards the edge of the frame esp the corners.
At some point, I will migrate to a full frame body and the artifact will be much more evident then
You can take a look at the vignetting on this link http://goo.gl/kTbGC . Mouse over the image to compare wide open with f/4.
At around half a stop wide open (APS-C frame), it's hardly something to stress over. If you really need to correct it, making an appropriate mask in Ps shouldn't be too difficult.
If you want to really go to town create your own profile at each aperture with the free Adobe Lens Profile Creator http://www.adobe.com/support/downloads/detail.jsp?ftpID=5489 it takes a little effort but the results can be well worth it, particularly if you use the lens a lot.