Jak's best attribute is his mobility. He can get in and then get out quickly. This also helps him maintain his range, which is where he's best. Stay out of melee range when possible.
I always use jak on online and it is a rare match when I dont come in at least 2nd, I can hit my level one about 75% of the time which is pretty good considering how **bleep** it is compaired to others. you just have to be able to use him corectly. it takes a dozen matches or so but he can be good as can any character in the game.