The original Poseidon Adventure was an interesting study of sociology, as it relates to disaster situations. The new version is an action movie, a special effects spectacular, with little plot and lots of eye candy. If you see it with this in mind, you'll probably enjoy it. Otherwise, you'll be disappointed.
For example... In the Poseidon Adventure, there's nearly half an hour of discussion before the main characters leave the ballroom. In Poseidon, there two lines of dialogue, after which the main characters just sneak out of the ballroom. The director thought he was just cutting out some useless exposition, and he was right - but that's only because modern audiences are dumber than they used to be, and prefer to get right to the action. The original Poseidon Adventure is a great movie, but it just wouldn't fly with today's moviegoers.
That being said, Poseidon has a lot of good directing techniques, especially when it comes to tension. I can't remember the last time I saw a movie that had me on the edge of my seat like this, wondering how the heroes were going to get out of their predicament. At several points in the movie, my heartbeat sped up to match that of the people on the screen. Note that I saw it on IMAX, and that probably made a lot of difference. I've seen several action movies on IMAX, but never one that seemed so suited for the format. If you have the opportunity, I highly recommend it.
A couple of problems... Poseidon was very predictable. I usually knew when a certain character was about to get killed, and I was only surprised by one death. Minor spoiler - there is a bit towards the end that was almost directly stolen from Armageddon. Also, I'm not quite sure on some of the realism, I think that more science-minded viewers will think it's silly... but this movie wasn't written for intellectuals anyway.
Despite those issues, I really had a great time. So if you like disaster movies, big special effects, and lots of tension, you should definitely consider this one. But if you're a big fan of the original, you might want to skip it.