OK I'll chime in with my 2 cents.
I use Slickedit 2007 (v12) on a daily basis, and have used it since v5. I also use Beyond Compare, especially in the past year while I have been working on some high-profile web applications.
Slickedit by itself is not a stellar IDE, because it attempts to be generic and as such cannot offer all of the bells and whistles of a dedicated IDE. (Take Flex Builder for example). But its real power comes in code manipulation and the macros you can write with its built in C-like language. I have written macros you wouldn't believe, as well as extending the editor itself with my own functions, forms, dialogs, and text filters (most of the editor's function is exposed in the included macro source). So I use Slickedit as a pure text editor in conjunction with whatever IDE I am working with (which is why detection of file changes is essential!). For quick coding, or when I need the code hinting, I will use the IDE, but when I want to really move text around, I go to SE.
But I will disagree with the other person regarding Diffzilla (slickedit's file diff tool). It is just too simple and, from my experience, awkward when handling UTF8 files. I gave up on it a long time ago when I discovered BC.
Beyond Compare has been an ABSOLUTE NECESSITY for my work ... I can't tell you how vital it has been when synchronizing hundreds of multilanguage utf8 files in several different code bases.
I cannot speak for Compare It as I have not tried it. But I will mention that some of the features it offers that BC currently does not have are possibly coming in BC3 (syntax highlighting, inline editing etc).