tl:dr Flex is handy, and so is roto. For me it's roto for 1/4" and is my primary 1/4" grab, and flex for 3/8", though it's not the first thing I reach for.
Long version:
My first non-traditional ratchets were rotos. I started with the HF Pittsburgh in 1/4" and 1/2". Rarely used the 1/2" (still have it since once in a while it comes in handy), but used the 1/4" a
LOT, especially in the engine bay of my old BMW, but for assembling stuff, etc. too. I didn't love the mechanism, so after trying several brands I wound up getting the 1/4" and 3/8" Icon.
I've found I use the 1/4" often, to the point that I got its Snap On brother since I love to run 2 ratchets. I like both a lot but seem to reach for the Icon more often.

I have a 1/4" LP90 flex head that mainly sits in the box. I don't use the 3/8" Icon roto as much (not sure why), but it did get used fairly regularly until I got the Matco locking flex.
Since the Matco came, I've found I use it a lot. Not as my fixed/regular ratchets, but a lot. I've done brakes on two of the family cars recently, and it's great for getting rear-facing caliper bolts without fuss, and the offset is really useful in a way the roto isn't. I've noticed it's good in spots like the rear strut bolts where the lip over the firewall makes a roto pivot challenging.

The roto doesn't clear as well as the flex-head here - even more useful when getting to the back blots of the PCV assembly on the family Volvo.
I've added a couple more flex head 3/8" ratchets recently (Ko-ken and a longer Icon), so we'll see how that fares. As a ratchet converting to a spinner, I still like my rotos, and for 1/4" the flex head isn't really competing with them at all.