Also, just to add to what Myke said, you can get through the main story of the game in around 20 or so hours; if you want to do all the side missions and stuff it's well over 100 hours.
If you like RPGs, open world sandbox games, and 3rd person brawlers, then this game is a blend of all three. You will probably enjoy this game.
If you do buy the game and enjoy it, please send a letter to Sega and let them know--especially if you're new to the series (like me). If enough new folks buy the game, maybe Sega will bring the 4th game here and actually put serious advertising around the game.
Sega of America, 350 Rhode Island Street, Suite 400, San Francisco, CA 94103.
A great thing that about this game is that most of the side missions have mostly unique stories. I have not been getting any sleep all weekend because I finish a side quest and, every time I think about going to bed, I want to do just one more. I've been up to 5AM Friday/Saturday morning and this morning playing this game.
There are also minigames peppered throughout the city, you can go in and play them almost any time you want. They're not as great as a full-fledged game; for example, bowling, it has a weird game mechanic, unlike many bowling games I've played. At first, these things are frustrating, but then you realize that you have to try to beat it.
The best thing about this game is the story. Yeah, graphically and mechanically, the game is not a AAA game. The developers probably had to be a little light on the graphics and such so they could get all the story and all of the side-quests into the game. So if you don't go into this game looking for another Uncharted 2 or God of War 3 (graphically), you won't be disappointed. The real thing that makes this game great is the story and the quirkiness of it.
I really hope you buy this game if you have the ability to buy it. This franchise really does deserve to be as big of a hit as GTA IV or inFAMOUS.