I absolutely love this album. I have all 4 previous albums from Nightwish, and enjoy them all. I know many people didn't like Century Child that much, but I think the songwriting on that album was very strong. The only criticism I share is that Tarja's vocals aren't as technically impressive on their recent albums as they were on Oceanborn.
I expected Once to sound much like Century Child. It does to an extent, but due to the presence of the choir and orchestra this is easily their most symphonic-sounding album. Listening to it, I couldn't help but visualize it as the soundtrack to some hugely epic movie. While I love the entire album, the standout tracks for me are definitely "Creek Mary's Blood" which incorporates Native American chant, and "Kuolema Tekee Taiteililjian" which is sung entirely in what I assume is Finnish (Hey, I'm not a linguist), and is an absolutely breathtaking track.
Very impressed. The only thing I'd change is that I wish Tarja would show off her voice a bit more as she did on the older albums.