Persona 3 and 4 are great games. They offer you an experience that no other game can ever give, because they're so unique. They were both pretty long to me, their stories are pretty long, and interesting. The new game+ option is pretty good, cuz you will probably want to do more stuff after you finish the main story.
The gameplay simulates a school life, and some kind of "dual life". You have your student life, but you can also explore dungeons crawling with monsters named shadows. The time in the game passes as the days go by, you usually have one/half month until the story progresses, so even if you're on New game+, it's probably not the kind of game you will speed run through.
The combat is pretty simple, It's turn based. In P3 FES you can only fully control the main character, however, you can tell the others what to do. You can directly control everyone in P4/P3Portable
P3 and P4 are very different from each other. I'd say P4 was a huge step up on gameplay features, so if you play P4 first, you will probably miss those features when playing P3...
Also, you should check P1/P2 if you didn't already. They are extremely different from P3/P4, but they're good, and both have their PSP remakes, in case you have one. I'd say more about the Persona series, but the post is already long enough, so just read reviews, try the games and see if you like.
I'd say a good estimate of gameplay length is at least 60 hours if you complete the main storyline for P3/P4. 100% completion takes significantly longer of course.
Boiled down to it's most basic, P3/4 features an overarching plot that advances slowly with short events as well as a boss about once per month. The overall plot is good but not expected to keep you occupied for the entirety of the month which is where social links come into play. Social links are side stories that the Main Character utilizes to get closer to certain characters in the game. I suppose the easiest way to sum up the storyline in a general way is to call it a journey through life. It's not your life nor is it a normal life, but it's an interesting one.
The battle system has a few quirks(such as extra turns for hitting weaknesses and all out attacks), but essentially it is just a normal turn based combat system. You'll probably have the most fun(and the most frustration) with the game's extensive fusion system. Your main character can choose between a number of personas (think skillset/jobs from other games), obtained from enemies. These personas can then be fused together to create new persona that retains some traits of the it's components (primarily, a number of spells can be passed on).
http://altogether.insani.org -> Visual novel goodness Maaya Sakamoto and Younha are amazing~ <3 <3
Alright man, I'll pitch for you. If you like stories in video games, you can't miss Persona 3. If you like combat that's strategic yet simple enough to grasp and master, you need to get this game. If you like character development, games that set a tone that grab you and don't let go, great soundtrack, and general style that is unlike anything you will ever play (even persona 4) you gotta get this game man. It's $20 bud, I'm telling ya, you will never get a better value for $20. (except for psn version for $10)
Give it a chance, you will like it if you like rpg's.