I really like this add-on. I use it everyday! Why is it so hard to find, btw, from the main add-on search page?
In reference to the prior review from Massimo advising that "Expression Search" doesn't work with Thunderbird 7.0, it does work. It's a bit confusing because after you upgrade to TB7.0 (at least on my MacBook running 10.7.1), it will complain that "Expression Search / Google Mail UI is incompatible with Thunderbird 7.0" with a little warning triangle. And, indeed, it won't work. But, do not fear. Just uninstall and reinstall Expression Search and it works fine in 7.0.I love the add-on, Wang Opera.
My only enhancement request is that I wish I could do negative combined searches: s:proxy !f:jeff (my example...show me all messages with proxy in the subject line, but not messages from jeff).
This review is for a previous version of the add-on (0.8.3).
I just change version 0.8.3 to be compatible with TB 7.0 yesterday, that's maybe why it's said incompatible at the first time.
For negative search, please try '-', eg: s:proxy f:-jeff, you can check the help page at Tools->Addons->Expression Search->Options->Help Next release I'll see if I can made the help page more easy to find.