I had never heard of this show until I browsing a "Top 100" list of shows, looking for something I had never watched before. This show was high on the list, so I looked at some reviews and thought it would be worth a look. Best decision ever. A series this long would usually last me a couple of months, but I watched it all in less than 2 weeks. I was addicted.
The show is amazing, it keeps the pace up for the first half of the first season, then slows down a bit as the aftermath of the initial "attack" (trying not to spoiler too much) starts to take effect. The second season is short, but a heck of a lot happens. I was scared that because it was cancelled, the shows end would be lame and open, but despite this it has quite a good ending. The premise of the show and the focus is one that it can't have a "everything is back to normal" ending, so the end that it is given does definitely give a sense of resolution.