NB: For those interested in this functionality but that prefer to apply it by default and blacklist certain sites, vs only having it applied to sites that are whitelisted, read the third edit.

All it does is set dom.event.clipboardevents.enabled to false, which can be done in a number of other ways without requiring another extension. Also, doing this is known to break functionality on Facebook and Google Docs, as well as possibly other sites. That said, I personally disable it as that loss of functionality doesn't really bother me, but the fact the author didn't warn users about it means either they didn't know, which is just plain irresponsible to not fully understand the impact of what their extension does, or they do know and don't care to inform their users. Either way, not exactly confidence-inspiring. Two stars just because it does do what it says, but I recommend doing it manually or with an extension like Privacy Settings (though, to be fair, that add-on doesn't warn users about this, either, but at least it can do a whole lot more).

Edit: Also, it should be noted that a big reason for disabling dom.event.clipboardevents.enabled is for privacy reasons, not just to allow pasting in blocked fields, because with it enabled websites can see what you're selecting and copying. By enabling it by default and only disabling it to allow pasting when such functionality is blocked, it's accomplishing the goal of getting around that blockage, but it's doing nothing to increase privacy. I realize that's not the intent of this add-on, but it's something to consider. Personally, I prefer to keep it disabled and only enable it when needed, which is rare. THAT ability might make it worth having a separate addon, as it would be much simpler for novice users than switching it as needed with Privacy Settings.

Edit #2 (in response to dev): It should be clear from my review, but be upfront in the description about what it's doing, and that this can be done in other ways, make it very clear about known breakages that are caused, to prevent people from being confused by them and wasting time trying to figure it out, and provide an option to disable dom.event.clipboardevents.enabled (enable the add-on's functionality) by default and quickly and easily switch it as needed, with the option to whitelist a site.

Edit #3: After a lot of searching and testing, I've found what I consider to be the ideal solution for privacy and usability: the Don't Fuck With Paste script (to be used with Violentmonkey, Greasemonkey, or Tampermonkey) by NeckBeardedDragon (based on this dev's code, which is of course based on another's). Not only do I prefer it as a script, which means one less addon and one less icon (it can be enabled/disabled as needed, though it's unlikely it would need to be, via the *monkey icon, along with any other scripts, which helps reduce clutter), but it works in a much better way IMO, which is that it works on all sites except those that are blacklisted, and by default it blacklists sites that are known to break due to what it does. So it provides the privacy benefits everywhere except those sites, where it would break them (and, of course, you can always remove them from the blacklist if you really want ultimate privacy and don't care about breakage). And it keeps you from even running into issues with copy/pasting, since all other sites are "fixed" by default. IOW, it's designed in a way that better protects your privacy and *may* break the occasional site here and there, in which case you can just disable it temporarily or add the site to the blacklist (which is the one weakness, since that requires editing the code which, while not difficult, is probably more than many people will be comfortable with). Inversely, this addon is designed as purely a means to fix the occasional site you run across that interferes with copy/paste, and does nothing for privacy, which, to me, is the wrong way to go about it for many reasons. Still, if that's all you care about (being able to block certain sites' ability to mess with copy/paste), this addon will suffice. But if you'd prefer to enhance your privacy everywhere, while accomplishing the same goal, I'd recommend the script. The sad thing is, this addon apparently used to work that way, and has since changed its strategy. It's too bad the dev decided to not only do this, but not even offer the option to the user as to which way they prefer to operate. And, of course, they still haven't added a warning to the addon's description about possible breakage.