First, get it. Like everything Valve makes, it's worth the money. It's got great puzzles that will make you feel stupid for overthinking them and it's about 3 times as long as the first one.
Second. It's hilarious. While there is no flat out quotable (The cake is a lie) internet meme, that I can see from this one, the dialogue from everyone is hilarious and magnificent. This is why games are made, and definitely how games get better.
Third, pay attention. Apparently, this game is set 200 years after the first one, which is nice. It also explains the basic technology for the surface required for a portal, as well as tells you alot about Aperture Science. Half Life fans, should enjoy learning about Black Mesa's "rival".
Fourth, get it. Again, I can't tell you how worth it this is. Get it on the platform it was made for (PC), and buy it through Steam. Barring that, I would have to say get it on PS3, since you get a copy on Steam with the PS3 purchase, and you can play across platform with your PC friends, then if no other option exists get it for X360. But either way, get it.
Fifth, get a friend and play the co-op once you finish the main game. It's extends the fun of the single player!
SPOILER ALERT VIDEO: This video shows some a few of the first co-op levels. They aren't hard to figure out, but if you want to see them fresh, then don't watch this. Also, this is displaying PC/PS3 crossover play