Once you install this extension, you realize how much you missed it before. All the times you half-consciously thought "if only I could use multiple criteria or a regular expression here, I'd find that mail immediately"...
It is a pity that a Global search reverts to the 'old' way of searching, without regex and with very simple boolean logic. But you can't have everything.
EDIT - the developer fixed the issue mentioned below within a few days after I reported it. Well done, excellent support! [There is one little glitch: if you put the Expression Search widget in the ToolBar, it is impossible to hide the Quick Filter Bar. Thunderbird simply doesn't allow it (in the menu: View... Toolbars... Quick Filter Bar). The empty bar keeps taking up space above the list of mails. (TB 31.5.0 on OS X 10.10.2)]
This review is for a previous version of the add-on (0.8.8).