Compels are used to increase the amount of story occurring, using a form of reincorporation of a player-chosen theme (i.e., an Aspect). This is often negative because we tend to tell stories where the rising action is composed of challenge and setback, but compels are not necessarily negative suggestions. Their purpose is to complicate or add nuance to the situation or to advance the action in a surprising direction. They can just as easily be positive or neutral from the character's point of view.
A space marine with the Aspect "A lover in every port", while trying to seduce someone, might be offered the obvious negative compel that a jilted lover shows up and causes drama. Equally, if that marine's trying to find contacts who can repair a beam rifle that's illegal to own locally, she might be offered a positive compel that she has a former lover on the station who brokers black-market services. Either way, if the player accepts the compel, the story has taken an interesting turn that the player is automatically invested in because it's tailor-made for their PC. That is the power of the compel, and it transcends simple negative challenges.
Of course, as referee I could just say the helpful contact is there. By offering a compel, I'm putting it in the player's hands to decide, and feeding the Fate point economy. Likely they'll take it, so the net effect is the same but I've added another Fate point to the economy. I don't mind being generous with opportunities for giving Fate points, and by being so I also fund their ability to turn down "positive" compels, increasing their story control (and therefore, investment). Do it enough, and the few times that they want to turn down the positive compel (and so have to pay for it) are more than made up for by the extra "free" Fate points they've been getting, taking care of the potential for it feeling unfair when they reject it. As a general rule in Fate, anything that increases the number of Fate points moving around (not just being given to the players, but also given and spent) is likely a good thing for the game.
One thing I like about the text of Diaspora is that it belabours the ways in which Fate can put the power into the players' hands (and, not-so-sneakily, take some load off the referee). One of the specific ways it does this is to point out that players can ask for compels. Hence, even a negative compel, when asked for, can be a positive thing as far as the player is concerned.
Best Answer
Summary: Negative aspects are easy "push button here" dispensers for Fate points, but spamming that button needlessly is boring at best.
Two things need to be kept in mind: drama, and the Fate point economy. Being stymied or drained of Fate points by the same problem over and over isn't dramatic or interesting, so don't do it. But the Fate points must flow! So here's how I approach compels in my games.
Encourage self-compels
Most of the time, the Fate point economy should take care of itself. If aspects are interesting and natural to compel, players will often do it themselves! Frequently my job is simply noticing that their natural role-play choices deserve Fate points, and handing out the points accordingly.
If players aren't self-compelling a lot it may be because they feel too overwhelmed by the challenges I'm giving them, so they don't want to invite more obstacles. In that case, I need to back off and give them room to make problems for their characters.
More often, however, it's because our aspects weren't written so it's obvious and interesting to compel them. That means we need to revisit aspect-writing and revise our character sheets accordingly, but in the meantime aspects such as Steers like a cow are great filler to keep the Fate point economy afloat.
Look for compels when pools run low
When players start to run low they often cast about for opportunities to self-compel for more points. Obvious weak spots like Limited battery pack are a useful tool in these situations, especially when their character aspects might not be able to provide immediate traction for compels.
If a player’s pool runs low and they aren't self-compelling, it’s time to start looking for ways to feed them more points. Compels are a great way to do this because invoke points don’t go into the player’s pool until the end of the scene while compel points are usable immediately. This is where obvious but not overly dramatic compels, like "your gun jams," can be great.
Compel when the stakes are high
Drama, drama, drama. When nerves are taut and the stakes are high, throw out compels which will really gum up the works. This is a spice, not a protein, though: use it sparingly and its effect will be all the greater. "Your flashlight dies" isn't very dramatic--unless it dies just as you hear a spooky noise while exploring the sewers!