Cissokho is an interesting left back offensively.
however, the real problem with him is his defending skills sometimes, especially in marking. I think this is his 2nd or 3rd own goal this season.

Cissokho is a very limited player. Technically he's a disaster (that's why how many times he clumsily put the ball into his own net) and he's not much intelligent.
As you said he sucks in marking but he can outmuscle and outrun most of players. The thing is he doesn't have the intelligence capacity to understand this so he doesn't use this as often as he should.
On the offensive side he's technical limitness prevents him to trick dribble an opponent, quickly turn sides, make some spins or whatever. He just uses head-through-the-wall style to charge through the left side and when he does, his crosses are awful 19/20 times. And he keeps his head down and doesn't look at his team mates when he's running like an ape along the left outline.
Even at Porto he wasn't some class. He played half the season there (15 games) and after a few interesting games Milan and Lyon started pushing for him and suddenly his price was as big as 15M€. Just a big transfer flop for Lyon.

Cissokho should be played as a winger: he crosses well and he has speed but defensively he is rubbish.

I wish he crosses well, but he doesn't. But hey, if that's up to Chelsea's standards, you can take him :)