Finished it last night at around 85 hours with everything completed besides the usual R&D1 "DO NOT EVEN TRY TO BEAT ME LOL" hidden ultra boss like Hitoshura in DDS. I'd imagine that if someone ran through it efficiently and skipped the final quests they could probably get through the game in about 70 hours. Though on the other hand people who immerse themselves in the roleplaying aspect of the game like TheTrin will probably hit 100 hours.
Game was pretty good and satisfying. Like other R&D1 games there are a lot of nice little touches in the gameplay/writing. Like how if you stand still for a sec the character will put his hands in his pockets. Then if you run in a direction, instead of the usual running animation, he'll have a special animation where he runs with his hands in his pockets. Also sub-themes/motifs are nicely placed throughout so by the end you're like "ahhh...cool". The characters were also one of the best groups ever since you spend so much time with them and they get a good amount of development.
The anime fmvs were awesome and a very good decision to include in the game. Otherwise it would've ended up like Baten Kaitos and little 16-bit style SD characters aren't really that great for conveying important story events. P3 has a good amount of fmvs and the studio that did them did a great job and they all look cool and are well directed.
Music is great but the caveat is that because the game is so repetitve, the music is so repetitive as well. A song might be great the first time, but after you've heard the first 30 secs of song X that plays when you move between room A -> B for the 300th time you get kinda sick of it. Tartaros music is like that too where you'll spend 4 hours dungeon crawling to a dungeon song that is a 30 sec track looped for the entire time. Definitely gets on your nerves after a while. But the good side to that is that when you 'do' hear new music it makes it that much more awesome because you've been listening to the same 4 songs for the last 20 hours until you want to shoot yourself in the head like the main cast and then suddenly there's something new and fresh and it's very awesome.
The gameplay is kinda like that too. You'll be doing the same things over and over and over for so many hours that when something different 'does' happen, it's totally awesome because it's fresh and new. That being said the day to day gameplay is still pretty interesting despite being extremely repetitive. Hanging out with communities is fun and the battles are AWESOME (boss battles are AWESOME+++). AI partners actually made the game more interesting. Crawling is fun in a Diablo-ish way and making Personas has to be the most addicting that demon creation has ever been in a SMT game or spinoff. I'd sit for 30-40 mins just making Personas sometimes and it definitely drives you to do the endgame stuff and make all the final ones.
Besides the long repetitive nature of the game, my biggest compaint is that the communities were all very fluffy. I kinda expected these active stories to have big bang satisfying endings to them, but community stories kinda go out in a unsatisfying whisper. I guess that's so you don't feel bad when you get to the very end and realize you won't be able to finish 1/2 of them and see their endings on first playthrough.
I still prefer SMT and DDS over P3 and DS Raidou, but all four games from R&D1 this gen have been very interesting titles that all felt unique and were worth playing through. Definitely looking forward to their next-gen work, though I guess they'll put out 1 more PS2 game next year before moving on.