We use a hybrid. We process a lot of incoming data and dump most of it into dynamo (it's ephemeral so the TTL feature is nice) and if we get capacity errors (Dynamo takes a while to scale up sometimes) we just dump our objects in the DB. The end result is we keep a huge amount of writes off our DB for processing incoming largish objects. The amount of data it stores would cost an arm and a leg to put into redis.

Granted, I don't think I'd want to use Dynamo for anything other than temporary data. Lock-in makes me nervous, and the way it scales up/down really makes it difficult to use it for hourly workloads...by the time it scales up we're close to done needing more capacity, then it doesn't scale down for like 40m after. We set up caps and the DB overflow machanism keeps things from grinding to a halt.