As kenman345 said, it depends on your expectation. It's not a typical shooter and the parkour gameplay is fun when you know where your're going. If you prefer typical 1st person shooters, I recommend FEAR 2.

Both are on Steam, and if you buy the boxed version game, you have to create a Steam shortcut so you can get the in-game Steam chat, but the game itself won't be transferred to your account so you can download it whenever you want.

It depends what type of game you're expecting. if you think of it as a, GTA type game, it's not what you're looking for. But if you like the concept and have the PC to run it, then its a lot of fun.

Conclusion: Worth $40

Yes, though I would advise getting the X360 version over the PC. For me, the controls need a hands-on console type feel to make the environment seem realistic (and I'm usually a devout PCer). The PC version is still good though.

I played the demo version on X360 excessively until I finally broke down and purchased the game. It may be short and it does have its flaws, but its concept, gorgeous graphics and integrative play style are definitely worth the cash. ...Though, if you're playing for plot, it probably won't satisfy. As G4 says, "It seems to clearly be fractured, as if the game was supposed to be longer, and there are incredible leaps in logic and nonsensical plot turns."

I own the game.

The graphics are gorgeous. It's a rush flying down a zipline from thousands of feet up. The music is also very fitting and cool. In general, the main strength of the game is its fresh sense of aesthetics. The concept, graphics and music all fit together in a nice package that's just fun to cruise through.

However, the game's production values far outmatch its actual gameplay. Hand-to-hand combat (while the concept of the game assumes you will try and avoid this all together, Metal Gear-style...) is extremely clunky and awkward.
Additionally, when it comes to actually realizing this idea of a "runner," this parkour expert/fast running, gliding babe who jumps and ducks and slides etc., it's actually pretty hard. I thought it would be (oddly) like a Tony Hawk game: after putting in a couple hours, you could smoothly create your own paths for tricks and what not. That just hasn't clicked for me yet, as it tends to be hard unless you kind of memorize the levels, which is a different vibe entirely.

All in all, it's a great concept, with a great presentation, but it fails to live up to its full potential. Personally I don't regret owning it, as it's still a unique game worth playing through, albeit a flawed one.

If you don't like the idea of spending $40 on great looking high-concept game that's rough around the edges, rent it.

To answer the first part of the question, I played Mirror's Edge on Xbox 360 and found it infuriating. Even if you push yourself to finish all the time trials and unlock the achievements the game offers, you'll still finish it inside of two or three weeks. You're much better off renting this one.

In addition, the story mode of the game is really easy and the time trials are really hard. This is one of the few games that I could have maxed out as far as achievements, but chose not to because it really was not fun. Think of Prince of Persia, except the game requires you to be faster and muc more accurate with your jumps and moves. At times it was as if the designers expected you to be some kind of super-player to hit certain jumps or beat certain time trials. In fact, if you decide to play the time trials and unlock the achievements, expect to be playing the levels over and over and over and over, because even the tiniest screw-up will tank your entire run. Couple that with some obscene loading times, especially in the time trials, and you have a recipe for a game that's really annoying.

Meanwhile, the story mode, while fun to jump from building to building, really isn't anything to scream about. The story itself is really bad, the writing is pretty bad, the characters are uninteresting, and while the level design is cool, artistically, the game is way too washed out for you to get a good sense of the city around you. Often environments just bleed together. This is especially annoying when you're trying to execute certain delicate maneuvers.

I liked Mirror's Edge some, but it's definitely overrated. This game is innovative, but not that innovative. It's fun, but it's not that fun. Save your money.