I keep getting 500 errors.
The first one I got was when I first activate the plugin. The site was completely inaccessible. This was due to errors in .htaccess.
Upon activation, it added the line # BEGIN W3TC Browser Cache on the end of a line that was already in my .htaccess. Moving this to a new line brought site back online.
I was then able to get back into my dhasboard and configure some of the settings for w3 Total Cache. Warily, I checked the preview mode option, but despite this, I hit 'Save', and again I had a 500 Error. This time it was because it had entered this line into my .htaccess:
BEGIN W3TC Page Cache core
Adding a # to the start of this line fixed the problem.