You passed the training and now it's onto the main game.
If you are heavily into story lines and plots as much as I am, let me start by saying that you should play Jak & Daxter first. The first game introduces some of the main characters for the series and gives you a general look at the world soon to come. If you don't want to play Jak & Daxter for whatever reason, you're in luck. The beginning of the series' plot starts at the very end of Jak & Daxter and continues straight into the beginning of Jak II.
The best way to understand Jak II and it's confusing but amazing storyline is to play it through a couple of times or watch the cut scenes over. Jak II takes everything you knew about the first game and does a 180 turn. Jak is now 2 years older and officially bad ass. After spending the entire length of the first game as a mute (except the occasional grunt), Jak is now fully capable of speech and one liners.
Jak II combines platforming with a sandbox type world which was inspired by the Grand Theft Auto games. As the game progresses, you meet new characters which are all interesting and have their own story to tell. You get surprises and chills as you wait for the next cut scene to drive the story forward. As well as having an amazing storyline, where would a game be if it didn't have good game play mechanics?
The game play is fun, offering you many choices on what you want to do and when you want to do them. You can pause from the main story and take time to collect precursor orbs or test out your skills in the gun course. You are given an eco rifle with four different mods; all of which provide four different styles in which you can take on enemies. Ammo is not hard to come by and most enemies die after a few shots.
Probably the most interesting game play element is the inclusion of Dark Jak. After harvesting so much dark eco (either from defeated enemies or crates), Jak has the ability to transform into a dark version of himself for a limited time. When Jak collects so many metal head skull gems, he can trade them in to acquire new dark powers (four in total). Dark Jak is basically a death sentence for any enemies who are hit by a dark bomb (the first power up) or get stepped on by Giant Dark Jak (final upgrade).
Whether you love or hate certain genres of gaming, you can at least appreciate how much Jak II has to offer. It's a fun experience and one that will be remembered for years to come by true hardcore gamers. If you haven't played this game yet, try and get your hands on it. Even if you just borrow it from a buddy, have a fun time playing it. It's difficulty isn't too hard and it offers some decent replay value in the form of Hero Mode. This is a harder version of the main game which allows you to start the game with all your weapons