I generally ride with either the snaffle or the curb rein, mainly
because one at a time is easier to deal with on a long ride.
I like being able to switch depending on the circumstances, although it
does take some practice (galloping up/down a hill because you've lost
both sets of reins isn't recommended).
It can be relatively severe, and does require a fair degree of care.
It's probably not the best choice for beginners, but my hands aren't
that great (have the video to prove it). It's the only bit I've found
that Bella likes that offers any degree of control, but I don't think
I would have gotten one if I hadn't had the opportunity to try it
first.
Helen & Bella (I'm fussy about bits)