If you've seen me play, you should've noticed I exclusively play in Open Tunings. I've been using them for about two years now and I've just found that it provides exactly what I want out of the instrument with almost no drawbacks1.
I mostly stick to 3 tunings - CGCGCD (which I call C2), CGCFCD (Csus) and CGDGCD (Orkney). I've tried using DADGAD before but I haven't found a particularly good use case for it, since it's essentially just Csus with an emphasis on D. I also occasionally mess around with CGDGBD, but again, I really only found it useful for one or two things.
I do a lot of work in this tuning, mainly because it's the primary tuning I use for Whiskey For Six gigs. It provides the chordal flexibility and bass fun that I usually like to use, with almost no problems.
It also makes tune playing a doddle in most cases. If you stick a capo on whereever you like, you're now in that key and most things will float over your fingers.
It's a bit mucky sometimes - trying to play a Bb chord is murky (though you can get away with it since nobody's actively listening to you), and anything with complex chord progressions tends to be awkward.
I've started to use this as my "I'm playing in a minor key now" tuning. Since a lot of my style revolves around the bass string, It's usually really important that I can hit the notes I need while still having access to the bass.
I also use it for pretty much every sad Celtic tune that has ever existed. Probably because all of them tend to use the tonic chord, then switch to 7th. Having that open F is so useful then playing a Bb chord.
Best examples are Bonny At Morn and Charlie David. Charlie David is a great example, because you can do a really nice G chord run that I noticed in Martin Simpson's version of "When A Knight Won His Spurs" that I felt obligated to nick.
I don't use this tuning enough, and I'm trying to work it into my standard toolbok. I first tried it when I was transcribing Roy Bailey's version of Palaces of Gold, but I just haven't found great uses for it.
In fact, none of the recorded material I have has even used it. I did do a YouTube video of Palaces of Gold after I learned it, and I have a couple of tunes in the works that fit nicely in Orkney that I want to try at some point but I haven't yet worked out all the kinks yet.
Well, it's a difficult one. I, like many other guitarists, began in standard (EGDGBE), but after listening to Sam Carter and Martin Simpson I gave them a whirl and found that I really liked the standardisation from string to string.
To be honest, it's probably because I really like playing a low C.