Having switched from my Pebble Time to an Amazfit Bip some time ago, I can tell you it's an okay replacement from my point of view.
The biggest issue regarding smartwatches in general is that their companion Apps are usually not available for SFOS. I don't know of any other smartwatch besides the Amazfish ones (with Amazfish) and Pebble with Rockpool.

The usability heavily depends on your use case. The Amazfit is quite restricted regarding the features (no Apps you can add, no media control) compared to the Pebble. If you are fine with the default features (stepcounter, hearth rate monitor, weather, alarms, timers, compass) that's good for you. If not, you have to deal with it.

Personally, I'm missing the media control feature (which I used a lot on my Pebble) and the water resistance of the Pebble (The Amazfish is protected against water splashes, but you should not submerge it into water). But as I already wrote, that really depends on your use case. Overall, the somewhat limited functionality is still better than no smartwatch with SFOS.