I'd say "use it as your database if you know your access patterns make it suitable/well-suited for its use as your database". Even then it will probably not be your only database — if it's part of your MSA/SOA.

I would not build in DynamoDB if you suspect your access patterns will drastically change over the lifetime of the application (or if you intend to, e.g., plan to build a data warehouse or something crazy with it).