i’ve only played offline, i played it like any other myst game. it’s long with the expansions, but i enjoyed it as much as myst or riven. my one caveat would be to look up where the journey cloths are in the starting area, it starts slow and i ended up stuck in the desert for too long.
i don’t know what the online really does because it didn’t seem to interact with the base game in any way. it was originally released with just the singleplayer, and there’s no puzzles that require other people or anything like that. in one of the expansions i guess there were supposed to be other people in the ruins, but it seemed like being the only one left would have added to the experience if anything. the online seems to be running again here https://mystonline.com/en/, but i’ve never tried it.