Firmware like Obervave, Hortator, X0X, and FMini-1, with their limited scope and greater depth I find to be more useful, at least as part of a bigger workflow.
E.g. it'll tell you how to activate the arpeggiator, as well as what an arpeggio is, what notes comprise each chord, scales, etc, which are all programmed into the firmware.
How long until we get that with a native support for changing firmware?
As in: preload ten firmware versions and switch between them, with persisted state?
but hook your FM1 to your phone and flash from there (the firmware hub)... very portable.
i like m-pyre a lot right now - https://historyofsky.github.io/m-pyre/
Additionally, understanding theory and improvisation is certainly a great thing to know if you're trying to make music, but you can understand a ton of things in the abstract and also be pretty terrible at applying it in the moment.
So if you want to get a device you can throw some hacked firmware on and have some fun, I'd say don't let your lack of theory hold you back.
"Useful" is a really subjective word when it comes to making music in general. Some template groove on a shit keyboard could be the base for a hit song (like Gorillaz' Clint Eastwood) while others don't make stuff unless in front of a modular synth with 100% control of everything.
Any instrument can be fun without knowing music theory, as long as you're willing to trial and error for a bit, until you find what sounds good :)