I owned the MachineDrum and MonoMachine and now have a couple of Octatracks.
The core of Elektron gear is the p-lock concept and their sequencers.
For me, the p-locks make a ton of sense for a drum machine. Punch in a run of hi-hats, then p-lock volumes and pitches for variation, add a slight LFO and you now have a track with some natural movement to it.
P-Locks also make a lot of sense for samples - lock the start, length, different sample, etc.
For a synthesizer, I found p-locks tedious. I ended up not using them much on the MnM. I also found that over time, I grew to dislike the sound of the MnM, no matter what machine I had loaded. Elektron envelopes, LFOs synced to the BPM, changes per step (yes I know you can slide params for smooth changes, but that’s more p-locking), all the things that are great for a rhythmic device or a sampler didn’t work for me on a synthesizer. Maybe I’m just old and used to more traitional ‘modulation over time’ using envelopes, LFOs, ramps, other oscillators, and performance controls like wheels and ribbons and such, but for me, the MnM never felt fun or engaging. It was never the first box I reached for when I wanted to create a sound.
For me, I have better sequencers and effects than what comes with any of the Elektron boxes, so the quality of those things was never a deciding factor. It was always about the sound engine. The OT works and sounds the way I always thought samplers should sound and work. It just took me a while to find it (been through the Emu Emax and E-III, MPCs, Fairlight, Akai S series, and random other things).
With all that said - these boxes are deeply personal choices. I know what works for me, but only after buying it and trying it out extensively in my setup. Sure, reading and asking opinions helped, but in the end the only way I know if something is a keeper is to get it and try it out for a while.
The advice of buying a MnM used is a good one.