"Channel Tools" plugin: How has this been useful to you?

I use it a lot, either to swap stereo channels, or else to widen (by applying a small delay to one side) stereo tracks or narrowing stereo tracks (to make room for things panned more L or R)

Stereo tools for me too, both fake stereoing a guitar track and actually narrowing true stereo tracks or instruments that are too wide for my tastes.

Widening

magik570
Is it mostly used on the master bus? 

That would not generally be appropriate.
The idea is focussed widening or narrowing the spread of a specific instrument within the mix.
For instance, many soft synths have a stereo output that may be too wide to sit well. You can use the channel tool to scale that down some.

I use it to shift a stereo track to the left or right.  You can't simply do that with the pan control.  For example I like to have my Hammond organ sound with a Leslie in stereo but not spread out across the whole stereo field. I like to have it sit more to the left .  With the channel tool, you can do that but still keep the stereo spread.

I use it to narrow stereo piano sounds that stretch from left speaker to right speaker.  Recently, I used Channel Tools on a cover of Drift Away where the stereo piano was interfering with the guitars that are panned left and right.  In the original the piano is located left of center.

It's my most-used of the bundled plugins and, aside from Pro-Q, the only plugin that finds its way into every single project. If it's got piano, organ, sampled guitars or synth pads, they'll normally all have Channel Tools on them.
 
Mostly, I use it for stereo panning as described by dlesaux above. But I also use it for both narrowing and widening on occasion. Usually not on the master bus, but often on sub-busses. I often automate the Angle parameters to give the mix some motion.
 
Yes, M/S balance can be beneficial on reverb returns to clear out the center for vocals. I don't do that unless it's a very busy mix that also needs a lot of reverb - and that's rare because the first thing I do with a busy mix is cut down the reverb.

The Console can change polarity on only an entire track. Channel tools lets you throw only the left or right channel out of phase, so you can cancel common elements to those channels while leaving non-common elements intact. Also with a stereo track where each channel comes from a different mic, you can tweak the delay time for one of the channels to reduce phase issues when the mics are at different distances from the source.

I did use it for adjusting stereo width but now mostly use WAVES S1 because it's got way more options to adjust the stereo field ...
 
BUT - I now use it a lot for quick timing corrections ... let me explain: quite often vocalists are just a tiny bit too early and "delaying" the take by just a few ms sometimes makes a huge improvement to what it feels ... Channel Tools gives you a dial where you adjust the amount of the delay without noticable overhead and it has what must be a logarithmic dial i.e. it's very sensitivity in the small range (so you can delay by just a few samples) ... it is also very helpful to illustrate to musicians that it sounds better if they don't rush the take ... and once they've heard it, they can often play it like you want it ...
 
likewise, you want the bass or the drums to be just a notch behind the beat ... Channel Tools in the bus and off you go ...
 
Unfortunately, using automation on the delay doesn't work too well because it can give you click/pops/artifacts even when adjusting delay in silent parts of the take ... so for final corrections of timing it still means split and nudge ...