Is it an actual MMO or more RPG-style without an emphasis on player interaction, market, parties/guilds and such?
In NGS, you can choose to play either alone or with other players because difficulty scales. Parties and guilds exist, and the toughest content can require premade groups as player skill is very important.
Does the game havea lively player base?
It peaks around 15k players a day now. The number generally goes down between updates (of course), but there will always be active communities or people to talk about and play the game with.
How about endgame? Does the game provide a reason to continue playing once you've maxed out, or is there a sense of "finishing" the game's content?
Endgame right now is hunting down powerful monsters that only appear during storms. It's a kinda weak endgame.
And lastly, would you recommend trying the game out? Why or why not?
Yeah, it's fun. There's a lot of people here who will say it's not worth it for various reasons, but ultimately the game is free, so there's no reason to not try it out, and you can always bail if you don't like it. Most people's issue with the game right now is there being nothing to do, but that has roughly always been the selling point of PSO2 (and now NGS) - updates come slowly and don't demand a lot of playtime, so you are allowed to play the game casually with other games.