Got to love it - and 2 ideas Rated 5 out of 5 stars
Using this addon for some two months now I got used to it and got to love it. I actually replaced the 'search' with this toolbar and only needed to enable it two or three times (I am on email on average 12+ hours a day, receiving close to 300 and sending around 60 emails). However, this also gave me the idea that this addon could also do what the 'unified search' was supposed to do - i.e. add an extra option to convert the filtering into a full-blown search.
However, my other idea is a much more significant one, and I believe it could lead to more significant uptake of this addon: make it work on the calendar tab as well. There is a vast empty space in the calendar toolbar, without a single button that I would actually use. However, if I want to search for an event (which is actually a filtering function) I need another 'row'. The ideal solution would be to get the calendar header (<<Today>> February 2013 ... [Day][Week][Multiweek][Month]) into the toolbar as well - there would still be plenty space for the filter. Seems to me pretty similar to the current addon - at least, from the user side :-)