Personally, I think that this is an extremely engaging idea. 
As for the other four deadly sins...
Lust: Bestows its host with impeccable charisma, confidence, and prowess. All of this comes with the cost that the evil side is incapable of self-control or prudence. Let's say a character sees someone across a bar that they find attractive, but they see that the person is sitting with her husband. The super-powered side kicks in, prompting the character to sweep the subject of their attraction off of their feet with inhuman suaveness. Perhaps the evil-side recedes just in time for the host to get punched in the face by their paramore's spouse?
Basically, the character gets turned into a smooth, charismatic adrenaline junky/ womanizer or maneater under the influence of the evil-side. It probably wouldn't hurt to add a violent streak too...in case the evil side doesn't get what it wants. The being of lust cares for the host so long as the host provides it pleasure.
Gluttony: Easy. Under the influence of the evil side, the character 'hulks out', gaining immense strength and senses, but have all of their senses replaced by an all-consuming hunger. The beast would consume anything; from rusty pipes to people. The non-evil side of the character could be forced to deal with the aftermath like in a werewolf movie. Transformation should be either involuntary, or at least heavily regretted after the fact. The evil-side would be the least intelligent of the bunch, with the host not having to contend with internal back-and-forths as much as a constant, gnawing hunger. The evil side might even be completely brainless.
Our Werewolves Are Different indeed.
Greed: The evil side possesses a brilliant intelligence and is capable of running mental-circles around anyone-even its host. It is a sociopathic being operating purely towards the fulfillment of its own ends, as such it schemes as much against its host as any of their mutual foes. As a being that is never satisfied, it will likely seek to control the entirety of the host's persona at some point. It should obviously have a tendency to horde things and a weakness for money.
Likely to be a Magnificent Bastard prone to muttering "Just as Planned" under its breath.
Sloth: Going with the 'evil occurs when good does nothing' interpretation of the sin, this being is actually the most benign of the bunch. It basically provides the host and those whom the host interacts with with an intense feeling of tranquility not unlike a drug. The drawback is, of course, that those who have felt the effects of Sloth pretty much want nothing else other than to keep feeling it or to feel it again. When removed from the being and its host they enter into a deep depression and perhaps become even suicidal. Have the character be a doctor or healer of some sort, initially using the powers to ease peoples' suffering before eventually realizing that this 'gift' is actually a source of evil despite their (and their Superpowered Evil Side 's best intentions.
Anyway, that's my two cents. I hope this proves helpful 
This could be a great game mechanic. I'm picturing it as equivalent to the Vigors in Bioshock Infinite. :)
I'd say I'm being refined Into the web I descend Killing those I've left behind I have been Endarkened
And after a certain period of prolonged existence with these Superpowered Evil Sides, the sinful traits could start to seep into the host's everyday personality, for better or worse. The character with Wrath as their evil side would become increasingly quick to anger over time in his or her day-to-day life, even in human form. The character with Greed will steadily become a little smarter, more manipulative, and more insatiable and selfish as time passes, the character with Lust becomes more attractive and confident, but also more insatiable and less mindful of others' feelings and relationships, and Gluttony, Sloth, Pride, and Envy run into similar long-term side effects.
edited 6th Jun '13 10:26:18 AM by Lennik
Wimpy Mc Squishy
I hate to say it, but this has actually already been done. The Exalted tabletop RPG has the Great Curse mechanic. When you build your character you choose one of the seven Great Curses, based on the seven deadly sins. When certain conditions are met, your curse activates and your character goes on some kind of rampage, depending on which curse you chose.
'Cross my heart, strike me dead, stick a lobster on my head.'
Apparition in the Woods
As I am generally wont to do, I would also like to add that the original scheme devised by the teacher Evagrius of Pontus was not of "seven deadly sins," but of "eight evil thoughts," or logismoi, which poisoned the mind and the body: Gluttony, wantonness, avarice, hubris, "sorrow*," wrath, boastfulness, and apathy.
There is a lot more nuance there than in the later versions, at least as they tend to be interpreted in less scholarly contexts. They also just feel a little less hackneyed... although, to be honest, this particular variation on the whole "shadow archetype as a source of power" thing is a bit old in and of itself.
edited 6th Jun '13 8:43:03 PM by JHM
I'll hide your name inside a word and paint your eyes with false perception.
As title says. I like both of these tropes and uniting them seems logical conclution. But figurim some of them isn't easy, so I decided to come here in hopes for some cool ideas. I've olready got three, so here they are for some feel of direction:
Anger: Somewhat classical mr. Hyde. Comes out becouse of fear, anger or any other sufficient reason. Can go in rampages, but prefers to show his hate for... well ewerything with cold sadism. This will most likely be protagonist. Cuz they're supposed to be most straightforward of characters. Right?
Pride: This one manifests as different person serving it's master/source. As pride it realy loves it's master... sometimes to grief for both of them.
Envy: Pick your abomination of choise and give it normal powered human side. Then imegine this abomination actually liking being human. Done.
Rest are not as easy. Actualy I'm stumped. Any ideas?