Good question! The Sentry is a wonderful character with a lot of potential although he has been mishandled in the past by writers who didn't know what to do with him. The worst stories with the Sentry were the ones where he was incidental to the storyline, during his time as a regular member of the Avengers. This meant his great power was either ignored or writers had to come up with reasons for him to not get involved. By his very nature, the Sentry should be central to whatever storyline he's involved in. But I'm getting ahead of myself.

Who is the Sentry?

The best way to describe the Sentry is this: he's a stand-in for bad Fan-Fiction authors. Robert Reynolds' one real power is the ability to rewrite history/reality to be more to his liking (i.e., the power of retcon) and his most fervent wish is to be the strongest, most powerful, most beloved superhero in the Marvel universe. He wants to be the golden child, friends to all the big characters, beloved by the common people, the best of the best even to the point of overshadowing the struggles and accomplishments of the ones he adores. So he remakes himself as the Sentry, rewriting history to do it.

When he's introduced, the world suddenly "remembers" him from key points in their lives. Reed Richards remembers that he was the best of friends with Reynolds and that Robert was even the best man at his wedding. Angel recalls that at a time of doubt in his early days as an X-Men, it was the Sentry that taught him a valuable life lesson about being a hero, making him the man he is today. The struggling Spider-Man can't believe that he had forgotten being instructed in heroics by the Sentry, and that the greatest accomplishment of his professional career was taking a photo of the him, winning Parker fame, accolades, and fortune. And the tragic figure of the Hulk recalls the "golden man" who was able to erase his tragic isolation, and bring him into the light as a hero for the world, but only when around the Sentry.

These seem like good things on the surface, but they're not. Reynolds has inserted himself into the most private moments of Richards' life. He's hijacked Angel's heroic narrative for himself. He's removed the heroic burdens from Spider-Man and the Hulk, diluting their characters (even going so far as to make the Hulk little more than his pet). This is the destructive nature of bad retcons in favor of an author avatar. Sentry is every bit Robert Reynolds' Mary Sue (or Gary Stu).

To make this explicit, we have the Void.

Who is the Void?

The Void is the opposite of the Sentry. He is the embodiment of the negative consequences of the Sentry's meddling in continuity. For every good that the Sentry does, the Void does an ill to make up for it. He brutally maims the Sentry's kid sidekick, Scout. He terrorizes Reynolds' wife, Lindy. He reaches into the heart of the Hulk and scars him worse than the Sentry ever "helped" him. He does these things because the Sentry needs him to, because heroes are defined by their villains. Reynolds wants the Sentry to be the greatest hero of all time, so he needs the most terrifying villain of all time opposing him.

It's important to note that while the Void is the opposite of the Sentry, he is NOT the opposite of Robert Reynolds. The Sentry/Void dynamic is a false dichotomy. By choosing to be the Sentry, he's also choosing the Void, but even if he did find a way to ONLY be the Sentry, as he attempted to do when Emma Frost "revealed" to him how the Void was just his imagination and he joined the Avengers as a regular member, the Sentry itself is still a villainous figure. It is only by rejecting his own power—by excising the Sentry and the Void from history and living his difficult and mundane life as an alcoholic agoraphobic, as he did in his original series—that Robert Reynolds can be heroic.

The dual nature of the Sentry/Void can be used to symbolize many things—the good and evil natures of humanity, mental illness (e.g., manic depression/bipolar disorder), substance dependency—but in the end, the manic extremes it represents must be rejected. For example, the highs and lows an addict feels when in the throes of substance abuse can only be resolved by cutting out both. In this sense, the Sentry should ONLY be used in a villainous capacity, even without the Void.

(Although the standalone THE AGE OF THE SENTRY mini-series used him to great comedic effect in taking the piss out of golden age Superman stories.)

What is the origin of his powers?

It's impossible to know. Because Robert Reynolds' powers allow him to rewrite history, this allows him to rewrite his own history as well. Whenever he reflects on his own origins, the details seem to change. Most versions involve some sort of super-powered serum that allow him to pull energy from seconds in the future, giving him "the power of a million exploding suns", but this is all for his own benefit. The flight, speed, strength, durability, and telepathy the Sentry gives himself aren't his "real" powers anyway. They're just what Robert Reynolds chose to give to his ideal avatar.

As I understand it, I don't see a reason to look past his introduction for his origins. In the original Sentry arc, we are shown an event where a scared, lonely man suddenly has a spark of power, and he uses this to become the Sentry. There's no reason to believe that the serum he drinks in the beginning of the story even existed before he "remembered" it in the first pages. Maybe Reynolds always had the power to rewrite history, maybe he gained it just then and chose to create the serum that would unlock everything else, but we've seen his power grow exponentially since.

Where is he now?

Currently he's considered deceased, but this is rather dubious (even more so than usual in comics). In his original arc, he resolved things heroically by rejecting the power, his last act being to erase all knowledge of his meddling with history and basically give up his dream. Since he was brought into broader continuity with the Avengers, his presence hasn't been as tidy. He alternated between being ignored, being "too crazy to fight", and performing ever-increasing feats of strength, such as ripping people in half (e.g., Carnage, Ares) and standing toe-to-toe with figures like the Hulk. He also learned the nature of his powers from an encounter with the Molecule Man, another reality-changer. It was around that time that Sentry got really crazy, seeming to merge with the Void (since they are the same character) and gaining the ability to rewrite history even after his death, instantly coming back to life whenever he's killed.

Imagine the surprise of readers, then, when he was killed during the events of SIEGE when Thor dropped a heavy object (helicarrier) on him and that was it. There was no giving up of power. Nobody's memories of the Sentry/Void were erased. In fact, the Sentry was celebrated as a fallen hero at his funeral, with characters (e.g., Thing) professing their love and affection for him, including past romantic entanglements (e.g., Rogue), when we had no evidence of this leading up to it. I think it's clear that the Sentry isn't dead so much as he wrote in a heroic "death" for himself and will return. With Marvel Now! ramping up, it seems we'll get to see his return, apparently as one of a new group of Apocalypse's four horsemen, composed of dead characters such as Daken, Grim Reaper, and Banshee.

Personally, I think the Sentry/Void has great potential as a character. I understand why many fans don't like him, of course; he has the power to totally rewrite your favorite character's continuity and that becomes the new status quo. That's terrifying. Hulk fans especially have a chip on their shoulder about the Sentry, which is why they were thrown a cathartic beat-down of the Sentry by the Hulk during the WORLD WAR HULK storyline. But I like him. I think with the right approach, he's a great addition to the Marvel universe.

In fact, I think he'd make a fantastic villain for an Avengers film. He's one part tragic hero (Reynolds), one part dark messiah (Sentry), and one part cackling super-evil (Void). That's a great combination, and the nature of his powers would allow them to go back into scenes from previous Marvel movies and reshoot them to include the Sentry changing things—saving Stark from Obediah Stane for example, or being the one to save the Hulk from General Ross and make him a super happy hero instead of Banner doing it on his own. It would be a fun way to show the strength of the Marvel film-verse's shared continuity by messing with it a little bit.

So there you go, for better, worse, or extra-worse, such is the Sentry.