I love it. It helps you split your work by scenes, organise all your research, etc. I like the fact I can then easily shuffle scenes around. I've also set up custom labels so each scene is labelled by POV (I've got multiple POVs in my book) and how finished it is. Then I can sort and just focus on one character, or on scenes that are incomplete.
The best idea is to get the free 30 day demo from the website and work through the tutorial to get an idea of the features.
Have this for about a year and love it. As well as what schmalex said you can also put photos/pics into the project- handy if you've drawn your characters or found pics on the net that resemble them. Also handwritten stuff can be kept with the ms e.g. I had drawn a very detailed timeline on A3 paper and just scanned it in. There are also folders to store characters and settings. Basically any details you had in notebooks/scribbled on paper can be kept with the project.
I also keep previous versions of the ms in the project itself.
I'd recommend the free trial. It does take a while to go through the tutorial but it's worth it (reminds me I should do it again because there are plenty of features I don't use at all.)
I love scrivener and it's great because you can set it up for what you need and have everything you need on one save file. I also save mine to Dropbox and backup to my computer so i can access the latest document from my pc or my netbook. There are loads of Youtube videos explaining it, which is quite handy, I don't use all of the features but still think it is great.