Spoiler Level: I'd say somewhere in the range between ther higher side of medium and the lower side of high.
I was debating not reviewing this episode until I saw the second half of the story, but there's so much to love in this episode I figured what the heck. River Song, for starters; I love the whole idea of seeing a future companion (hey, she got to travel in the TARDIS in this story, so she's an official companion in my book now) and that we're getting to see her story from the Doctor's perspective and not from River's. The 11th Doctor's reaction to her, which is much more humorous than the 10th Doctor's. The interplay between Amy and River; it's always fun to see companions from different time periods banter.
I also feel like the romantic relationship between the Doctor and River works better for me than his relationship with Rose. Now I'm not hating on Rose at all, and in hindsight I'm quite okay with him having fallen in love with Rose and their love being the light that put him back on track after the Time War; but at the time I just felt she was starting to steal the show from the Doctor. By contrast, we meet River and immediately learn she's going to be special to the Doctor, and now we have the intrigue of wanting to find out how and why. Plus, by being a semi-recurring companion, she doesn't overpower the show. So I'm really looking forward to seeing her story unfold. I hope she ends up becoming his wife out of a sincere love on the Doctor's part, and not out of some cheat as in "The Doctor's Daughter" or plot device as in "The Aztecs." (Do you realize we know of the Doctor having gotten married three times now?)
I also felt the Weeping Angels were very well handled. I have to admit, although I feel "Blink" is the best Doctor-lite episode made yet, I didn't really think there was much more that could be done with the Weeping Angels, let alone as a two-parter. But this story has been great at expanding their mythology, as Rich put it after we watched it. They did a good job creeping out Mylene as well, although I think the monsters to creep her out the most this season has been the Smilers.
Looking forward to Part Two. I'd like to learn more about the hints they dropped about River, but I'm betting they won't be answered until later seasons.