Early S5 indicators seemed like the show was finally going to get it right. Alas, the decision to incorporate Anna's RL pregnancy into the show has proven an unmitigated disaster, and since they seem bent on muling through this arc instead of conceding defeat, it's only going to get uglier.
As much as many people here don't like the pregnancy storyline (and I have to say, it's definitely not the way I wanted it to go despite having some fondness for DL) I really don't think it's been an unmitigated disaster and see little evidence that it's influenced the ratings to any detriment to the show. The low ratings seem to me to not have followed any D/L or pregnancy drama heavy shows, although do feel free to prove me wrong! I don't think I've seen any indication that the relationship stuff has affected the ratings at all, either positively or negatively and the show had already dipped significantly before the pregnancy storyline began.
Having said all that the season did start stronger than any before and it's a shame it's not continued. Personally I think the first few episodes were definitely the best but there has been a clear correlation between the drop in ratings and an increase in competition with other shows. As for 5:13 I've seen even die hard NY fans say they recorded it this week because they were already into Lost and wanted to watch the second half.