Is it a linear experience where you follow the main story or does it have secondary content to go through?
Well I haven't played a ton yet, but so far the world is open and there is side content to explore, but the main story will keep you moving forward. It also lets you piece together the clues on your own, without holding your hand.
That sounds great, but judging from what you've seen in your playthrough do you think these things impact the story or the gameplay in a substantial way or are they just secondary characteristics that add little to the game ?
They impact the game via stats. Your sanity for example, is based on which background philosophy you've chosen. Age plays a part in how skilled you are - the older you are the more skill points you gain via experience, but you lose attributes like Physique, or Senses. Your background job description plays a part in your skills as well, what you'll be skilled in and what you will need to work on or leave unskilled.
The gameplay footage i've seen did kind of scare me since the combat seemed like such a slog, does it take up a lot of time from the game and if so is it really worth that time?
The game is not a slog, it's slow but it's not too slow. Combat, I had to run after killing one raving lunatic because the game said "the escape route is open!" and i couldn't possibly take down the other lunatics, so I ran. I still gained XP and had an impact on my stats from the combat. But no, it's not too slow, at least not for me, but I should let you know that I'm a slow player in general and don't mind this kind of thing.