I assume you've already seen the film. Most people feel it's a great film, and I won't argue there. If you want to buy it, make sure to buy it on Blu-ray. I've also owned it on DVD and the Blu-ray simply looks so much better and has tons of extra bonus features.
It's available in a number of packages (single disc, multi-disc, Blu-ray plus DVD plus digital download, 3-D, etc.). Is it worth it to have all the extra bonus features.... that depends on how much of a Titanic buff you are. Many people will simply want the movie and won't watch the bonus features at all or more than once. For them, I'd recommend getting the single Blu-ray disc. This particular multi-disc set gives you the movie and a LOT of bonus features that are very interesting about the Titanic ship and the making of the movie.
If you find this on sale it's a great value. I cannot comment about the 3-D version as I've not seen it. It wasn't originally made or intended that way so it's likely a novelty experience.
I'd also recommend you watch the older black and white film, "A Night to Remember", which is arguably the best Titanic film of all time.