I love the smut and the situations (reminds me of a certain lusty month in format) but their interactions are perfect. I love the slight reluctance in Snape and Remus' naturally adventurous side. And Snape enjoying himself is fantastic to see. I love this bit: Severus buried his face against Remus' neck to muffle his confession to both his own ears and Remus'. "I enjoyed it." Squee!
The description of each is fantastic- I especially liked the descriptions during the cock ring/collar one. Ooohhh and the bathtub! And how could I NOT love Archie/Hornblower? Yum! (even though book!Archie isn't at all like movie!Archie)
I can't wait to see what the men get up to in the second half!