Invaluable and indispensable addition to Mail.
You do, however, need to check for updates whenever Apple updates Mail and disables bundles (it doesn't update itself and plist is difficult to find).
Once you use it you will wonder how you managed without it!

I cannot overstate the impact of this little program on Safari. With AcidSearch Safari becomes the best browser by far. I then had to remove it to prevent 3.2 from crashing continually and, without it, the crippling inconvenience caused by Apples greedy exclusivity deal with Google, renders Safari a hobbled parady of what it ought to be.
Come on Steve Jobs! How about putting the user before profits and incorporating multiple search engine options. It's how it should have been from the start.
I, for one, have reinstalled 3.1 with AcidSearch again. If this ever fails, I'll be switching to Firefox before resorting to Safari 3.2 or its emasculated successors.