I heard that you have to load all your synths on with a computer and can't built them on the device itself like you can on the op-1

Yeah, though it's quite trivial to upload sample sets or tweak sounds in an editor (it uses Nova engine, similar to Ultranova/Mininova). Moreover, the knobs on the top allow some advanced tuning anyway, so even dull stock sounds can be made pretty cool. It's powered by 6 AAA batteries as well and has a speaker, so it's a pretty cool instrument for vacations and travel.

You load synth presets using a computer app which seems to be some sort of electron nonsense. It looks pretty bad.

There's also an iOS app called MIDISynthControl which can be used to edit the presets when connected to the Circuit over USB.

It uses WebMIDI, I think, so you don't have to install anything on your computer or phone to edit or update patches.

Yeah but you can't come up with a new sound on the go. One of the benefits of the op-1 is you can build an entire song on it.

Agreed; the point was simply that needing a second computer in this case isn't as bad as it sounds.