Some quick impressions from a short time with Final Fantasy Record Keeper:
1. it's Final Fantasy. It's got all the things you remember: the music, the battle system, the terminology, the fan-favorite cast
2. it's free, but it's stamina-based. I was able to go for a good bit (maybe an hour? about 4 or 5 full dungeons) before hitting that wall, but I've hit it.
3. it seems too easy at first, which compounds the next two issues
a. there's an optimization button for nearly everything, from party to equipment to skills
b. there's an auto button for battles
#3 seemed to be the biggest issue for me. Between an ever-optimal party and auto-attacking simple mobs, I only ever actively played anything when I came to a boss. Even then, I'm confident I could have just left it on Auto and came out with nearly the same results.
I mastered each dungeon I came across and seemed to be leveling at a pace to continue to do so. I wiped when I tried the daily event on Normal, which had no indication that it would be a scenario where the mobs take damage in the double-digits while they one-shot you. But then I mastered that same daily event on Easy.
But it's got some good nostalgia going for it, and if you ever wanted to see Kain hang with Cloud, this is it. It's perfect for when you can only pay periodic attention to what you're playing. I'll stick around with it for a little while longer, but my inclination is that it's going to get pretty grindy (considering the gulf between Easy to Normal events) which will exacerbate the Stamina mechanic. We'll see.