A flawed diamond Rated 5 out of 5 stars

In general, I like this a lot, mostly for the regex searching. The only criticisms I have are about the UI.

The UI problems come in two sorts. The first is those that have to do with dialogs. For instance, when you use the keyboard shortcut to enable or disable FAYT, a big dialog pops up in the center of the screen, making any further actions impossible until you dismiss it. Of course, dismissing a dialog isn't very difficult, but it would make far more sense if the notification were a small thing that didn't require any interaction with the user. The search panel has exactly the opposite problem; it vanishes as soon as it loses focus: when you hit "search", or click on anything that isn't in the search box, or switch to another window, or hit Alt twice, or move the scrollbar, etc. So if you want to modify your search you have to bring it up again.

The other UI problems are mostly because it really shouldn't be one extension. It would make more sense if the find-in-page functionality were separated from the web search functionality, and possibly if some of the web search functionality were further subdivided, so users could download only those features they want. This would improve the user interface immeasurably, mostly by reducing the number of settings you have to figure out. (There's also a minor bug here: some of the settings don't always work; occasionally, even if you have an option unchecked, the extension will behave as if you had it checked.)

Don't let any of these criticisms dissuade you from downloading this extension. It is overall extremely useful. It is only because it so closely approaches perfection that its flaws stand out.

EDITED: After reading the response, I decided the developer's right; given how much I love the regex searching, this extension deserves five stars, not just four.

This review is for a previous version of the add-on (2.13). 

Edited: So my "subtly laying out my case" worked, thanks! :)

I respond to your points below:

1. FAYT modal dialog. It is meant to be used only occasionally, so modal is no big deal. You might have found it bothering because you're new to the addon and would like to turn it on/off multiple times to compare. But once you decide, you'd rarely need to use it.
2. Search Panel gets dismissed. Since Search Panel supports FAYT by default, it is assumed that once user press 'Enter' or click 'Search' button, that's the final search user wants and the dialog should be dismissed.
3. Splitting into multiple addons. The main issue is performance. Splitting is only advantageous for users that want only one major feature out of the many FS offers. For the majority of users who want at least two or more major features, having to install multiple addons that should otherwise share memory/event handling would be bothersome and performance-impacting.
4. Too many options in UI. Well, yes, it's true. But I've made an effort unmatched by the vast majority of addons out there to make it easy on users while still supporting the huge number of features/options that they wanted/suggested. I provided a blue '?' for nearly every option, I provided easy to use FAQ and lengthy Help documentations on my website, I made multiple demo videos that took many hours as I tried to show the maximum using the minimum time. Just for a comparison, TabMixPlus has a huge # of options too, but is there as much effort spent on making things easier/clearer for users?
5. Some options still work after being unchecked. I'd like to know which one(s) are you referring to. I kinda suspect you might have misunderstood them instead, because I've specifically checked every option before I released the latest versions and they all worked.

In summary, I feel that some of your points are good but nothing better could've been done to cater to all users as I explained above. Other points are more personal preferences that no addon can satisfy everyone. As for the option bug, please pinpoint it to me so I can check/fix it. But I do suspect (as I said above) that it's a misunderstanding instead of the option not working.