Definately worth at least a rent if you're a fan of Dead Space.
In the main story mode you play a few different characters as you progress through the events that happen upto the start of the first Dead Space game. Dead Space Extraction puts a surprising amount of time into the story and characters, with almost as much character interaction as actual shooting, and luckily it pulls it off well. Though if you're just here for the shooting, all the dialoge slows down the pace of the game and may put you off.
The innovative shooting mechanics from the first Dead Space translate well into the game. The weapons all have an alternative fire mode, accessed by tilting the wii-mote onto its side, and strategically taking down an enemy by shooting off its limbs gives it more depth than a typical point-and-shoot game. The weapons feel punchy and extra abilities like stasis (to slow things/enemies) and kinesis (to pick up/throw items) are mixed in with the shooting.
The presentation of the original Dead Space, with its impressive lighting and sound, is pulled off surprisingly well on the Wii. It has some of the best graphics I've seen from the system and strong sound effects. The framerate suffers at times with lots of enemies on screen, but it's a minor issue and doesn't effect your aiming at all (it would have been a bigger issue in other genres that give you full control of the view and character).
As you progress through the main story, you unlock episodes from the Dead Space comic presented with full voice acting and some basic animation, which are a nice extra that I was always keen to watch after unlocking. You also unlock levels in the challenge mode, in which you go through a level fighting off waves of enemies. This is ok, but feels a bit hollow compared to the main story mode and highlights some of the shortcomings of the gameplay - it just isn't as thrilling as the combat in the original Dead Space and doesn't deliver the same sense of panic. I think this is mostly down to the fact it's an on-rails shooter so you only have to pick off the enemies you can see on the screen; you don't have to worry about what's creeping up behind you because you have no control over your viewpoint.
The main story mode lasts around 5 hours, but the extras help add an extra hour or two. Though once I had played through it I felt like I had experienced all it had to offer and doesn't really feel like it has much replay-ability. As such it makes an ideal rental/budget buy for people interested in Dead Space.