Since Day-O isn’t going to be updated for Yosemite, this has replaced it on my menu bar. So far, so good.
To be honest, Apple had already (sort of) “Sherlocked” Day-O by including a new option to show the current date on the menu bar (which is what I need the most), but there’s no way to customize the date format (I get the day and the month, which take up too much space), plus being able to view my calendars at a glance is a substantial upgrade over what Day-O provided. I won’t miss it.
Update: I just got a suggestion to check out TinyCal as well, which seems rather nice too.