A great premise, complemented by good gameplay, but everything else is severely undercooked.
The game is set in a 1984-esque future. Society is locked into a tight survaillance grid, and the only way to get data around without the government knowing is to employ runners (basically a bunch of 'le parkour' masters that use city rooftops and other inaccessible areas as a highway). You are one such runner, Faith. When your sister gets framed for the murder of a major political figure, you start digging around in order to clear her name. What you stumble upon is something I'll let you discover.
The premise sounds pretty good, eh? Well, sadly, the story never really takes off, never becomes really interesting. In fact, the story consists of a few anime-style cutscenes that tie missions together. What's worse, the rather limited cast of characters has repercussions on the sense of scale of the game's central conflict. So to sum it up: story/characters/setting are undercooked.
I mentioned missions earlier, let's talk about that. The first-person platforming feels great, and because the main character is quite fragile in fire fights, encountering hostiles can get tense (even deadly). So it's best to run from them. Indeed, some of best moments of the game come in situations where bullets are flying every which way while you try to figure out quickly where to go.
However, there are two problems regarding missions. One: mission design is linear, so there's usually only one right path. Two: missions are too much alike, so the game feels repetitive. You get to run around a few rooftops, fight a few police officers or swat guys, go inside some building and solve some lightweight puzzle, and run around a few more rooftops. So, as you can see, the good gameplay is not complemented by good mission design. Again, I'll use the word 'undercooked', because it sums up this aspect of Mirror's Edge rather well.
But there's more to this game than the single-player. We get two extra game modes. First, there's Time-Trial. As the name suggests, your goal is to set the fastest time on the various maps on offer. In addition, your results are uploaded and ranked, and you can even download other people's run and race against their ghost. Second, we have the Speed Run mode, which can only be unlocked by finishing the single-player campaign. The name pretty much says it all, so there's little use to go into details. Both modes are worth playing, and an inspired addition to the game.
Moving on, I'll go over a few secondary elements. First off, graphics and sound are top notch. Thanks to the unique visual style and the amazing sound effects, good voice acting, and great soundtrack, the game ranks high on my favorites list in terms of audiovisual presentation. And it isn't very taxing on your hardware either. In fact, the game runs silky, with no slow-downs, bugs, or crashes. It's certainly a very polished game on the tech side.
So, is Mirror's Edge worth a look? Considering that it sells for around $15? Yeah, sure. Just don't expect a masterpiece or something.