The app I've been working on doesn't use replicas for read scaling, just cache-money (ergo, activerecord). We're partitioning for write scaling but no slaves (well, we're doing master/master replication but not for read capacity).

After having had to contend with replication lag, lots of instances spinning and related headaches for years I'm pretty convinced that scaling reads with binlogs is in my past for good; it's way more expensive than having a good write-through cache.