I saw the movie today and thought it was fantastic. Put some closure to the series (although I hope they do a trilogy...).
I'm curious what other sci-fi movies you've seen in the last couple of years that you think are better than this one? War of the Worlds wasn't even close IMHO. Star Wars, maybe, but only because of the history; as a standalone, forget it. Hitchhikers Guide? No way... Can't really think of too many other sci-fi movies recently, sadly enough.
That said, it definately wouldn't hurt to see the series (or at least the first few episodes) first. It's hard to get a lot of charater development into a 2 hour movie, especially with the vast number of characters on the show/series.
Overall, I think it's an excellent movie, better than 99% of the other stuff that's released.
CW