I've done the overnight cruise going the other way, viz. Stockholm to Helsinki. I was weighing up my options between this, a train or a night in a hotel followed by a flight; the overnight cruise was the most economical, particularly when booked close to the date of departure.
Cannot quite remember the operator (without pouring through all my photos); I think it might have been Viking Line. I took this ferry in winter, so when I boarded the ship, even though it was relatively early in evening, there was no light left (and it was even drizzling). It's a fairly long cruise in a way, as in you do spend quite a bit of time on board, particularly when there isn't much of a view (except after first light, maybe). I think it was something like depart Stockholm at 1800-ish and arrive Helsinki at 1000-ish.
I had the cheapest inner cabin, which was comfortable enough. Nothing snazzy, but it was clean, and I had a separate shower and toilet in the bathroom (and the shower was decent pressure).
Can't quite remember the dining situation; I think it was a buffet style. Not glamorous but all clean.
@Stealthflyer 's "feeding time" remark probably rings true. I think I bought some food from a cafe/store on board for dinner and breakfast, rather than dined at the onboard restaurant.