Yes it's completely safe to use, the only real downside to enabling it everywhere at all times is that if you have content being loaded from another source (aka loading an insecure image from an http protocol on https), it won't work, so some sites may behave unexpectedly. If that happens to be the case however, you can just toggle it off easily by clicking the icon. Pretty handy and powerful little extension for some extra peace of mind.
EDIT: An example of the type of error you'd typically see is something like this:
Code
WARNING: Mixed Content: The page at 'https://-redacted-' was loaded over HTTPS, but requested an insecure image 'http://-redacted-.png'. This content should also be served over HTTPS.