I think this is a good idea, as on many of these sites an HTTPS connection is not the default or is only used during login and then the rest of the session is unencrypted. However, while there are some good aspects of this beta of HTTPS Everywhere, the Firefox extension still comes up short in a few aspects.
First of all, despite the name, it is hardly everywhere. Only a handful of sites are enabled to use HTTPS Everywhere, and out of the nineteen sites currently available to be surfed using HTTPS Everywhere, only half can be considered major sites that most web users are apt to visit. Of course, I expect this list to grow, and information on writing one's own rulesets for HTTPS Everywhere are available online.
But the biggest problem for HTTPS Everywhere is that on some sites it can block or prevent access to commonly used features. For example, when I used Facebook with HTTPS Everywhere, the integrated chat feature in Facebook was disabled.
And it wasn't that easy to get around this, as HTTPS Everywhere wouldn't let me manually type http:// in the address bar in order to get around the block. I had to instead either turn off HTTPS Everywhere or disable Facebook as an option in the extension's options window.
Still, this is a beta and hopefully its functionality will improve, as I do think it can be a very welcome option for web surfers. I especially liked how it made sure that every one of my Google searches was done within a secure HTTPS session.
The beta of the HTTPS Everywhere extension for Firefox is free to use and can be found at www.eff.org/https-everywhere