Short version - different, very different.
Longer (and hopefully more helpful) version - It seems to be an album that's really splitting opinion and there doesn't seem to be a lot of middle ground.
There's a lot going on, more than I personally think anyone can pick up in one listen. It's NOT a return to the Kill/Ride/Master days, the most obvious giveaway being the total lack of solo's. What it is is kind of hard to explain actually so here's something I jotted down for a mate as first impressions.
Fast, really really fast, riffs everywhere, heavy sound. Snare drum sounds like a garbage can lid (I like it as it seems to fit the sound, you may not), entire sound production tries to make it sound like a jam session and just about gets there. Needs a good stereo as it shows up any imbalance in the sound, especialy at the top end. Good value with a DVD of live playing in rehersal thrown in and a key to the on-line metallica vault.
Basically I've had it in the car since it came out and right now anyway it's easily in my all-time top ten. In the Metallica list, I don't know, I'd put it about midway between Master and Metallica (so MOP 1st, SA 2nd). There's tracks here which seem almost Pantera or Sepultura like, but it never feels derivative. As one review I saw put it, while the earlier stuff was controlled violence, this is uncontrolled agression and that's about right.
Now the bad - as I said, you either love it or hate it, and I'd suggest trying to get a listen to it before buying. Lyrical themes are (not surprisingly) agressive and angry, if you can relate (as a worrying number of my friends seem to) it's great, if you're a mellow person by nature I suspect some of it may pass you by.
While it sounds NOTHING like Load/Re-load (which I always feel get a bit of rough ride, there's some great stuff on there, just not the overall quality of everything else they've done IMHO) it's the same idea with Metallica going off and doing something different. This time though they might just have cracked it. My suggestion - go find a friendly record store and have a listen (tracks 1, 4 and 9 are a good bet) and see what you think.