I have no personal objections to Darwinism, but as a theme for an album I did find that it was not nearly as compelling as the sort of topics that Tuomas has written about in the past. I'm definitely hoping for a new lyrical direction on the new one.
As for my overall evaluation of the album, I like it (as I do all Nightwish albums), but I agree with Katt that it feels similar to stuff from their prior albums in a lot of places (Shudder -> Dark Chest, My Walden -> I Want My Tears Back, Alpenglow -> Last Ride)—though I don't know what you think sounds like Sahara?
I hope to hear a bit more of a new direction from them on this album rather than EFMB part 2, though I imagine I'll enjoy it regardless, seeing how much I like all of their post-AFF albums.