Kessel is certainly a flawed player, he sucks defensively, is not physical at all, and tends to shy away from the danger areas. However, he is absolutely dynamite with the puck. Great speed, unreal shot, very good hands, very good vision. If he's on a line/team that doesn't have the puck a lot, that's not good, because he's not a star who will retreive the puck himself (like Crosby, Datsyuk, Toews, etc.), and he's a liability defensively. However, if he's on a line/team that DOES have the puck a lot, he's a massive positive, because he's so deadly offensively.
We're currently a weak team that loses a lot of puck battles, a team that generally does not dominate the puck, and I think this dramatically hurt's Kessel's even strength effectiveness. I'm not convinced he has a large positive impact at even strength right now (great scorer, but his negative defensive impact is amplified by our weak team in general). BUT if we ever build a really good team around him, his impact will be very positive. He could be like Heatley on the Heatley/Spezza/Alf line, Heatley was a one way player too, but it didn't matter because that line always had the puck.
Long story short, I want to keep Kessel, because even if his even strength impact is a bit "meh" on our current team, it could be great if we can improve our team substantially. The only thing I'd be worried about is losing him for nothing as a UFA. That would totally suck, but at the same time we would NOT receive a Kessel quality player back if we dealt Kessel, so we should only consider dealing him if management feel strongly that he will bolt.