Jak II is maddeningly hard but that won't bother you one bit.
The plot is amazing. Jak escapes from prison and so now you have to get revenge on the evil (but somewhat nobel) Baron Praxis. You team up with a rebellion set on killing Praxis but also keeping the third party, the Metal Heads, out. The plot twists are great and the best part has to be that the Metal Heads turn out not to be so much as a third party but just as much part of the game as BP.
The character depth is great. Daxter is no longer an annoying prick which is an improvement. Jak now is a total badass who's badassery rivals that of da Terminator rather than the pretty boy he was in the first game. Secondary characters' depth is just magnificent. From the half-hearted nobleness of Baron Praxis to the outright ruthlessness of the Metal Head Leader, from the Mafia Don-esque persona of Krew to the soft-spoken Torn, they're all memorable in their own way.
Now the one thing that'll make you love or hate this game is the difficulty streak. It will get hard and believe me this game is just one rung down on the difficulty ladder from Ocarina of Time's Water Temple. Luckily the game makers realized this and allowed you to quit the mission you're stuck on anytime.
The game seems almost like a movie which reminds us of Metal Gear Solid. The difference: the cut-scenes aren't long which makes it better. If there was any game I could point out as having extreme potential as a great movie it'd say Jak II.
The gameplay itself looks like a cross between Halo, Banjo-Kazooie and Grand Theft Auto. Which is good because honestly it's a great game. It's a technical masterpiece and I would strongly advise renting (or buying) it.