Quick thoughts: speedy, nice throughput, good read/write speeds generally, but the query optimiser still needs some work.
More detail...
So, it's a pretty nice engine. Scales well, seems fairly bullet proof. You can alter instances and they go up and down in size nicely, reboots are handled gracefully (although it doesn't seem to give estimated timings, which is a mild irritation).
The optimiser seems to get a bit stuck on understanding what indexes to use for queries that involve multiple joins or more complex data structures. My guess is that this is because under it (from what I understand), it's basically DynamoDB with clever bells and whistles (if someone knows more or that that's incorrect, please feel free to jump in). Mostly though, it's quicker for the majority of queries.
Backup management is lovely, caching system seems pretty performant from current benchmarking.
The metrics panel is also pretty sweet for monitoring the things that you're going to care about in DB-land.
I'd say if you're running something sensible sized, and your data is relational but not massively complex, it's a pretty solid solution which I'm sure will only improve over time. That said, there are a few edge cases where you're going to have to do some work on your queries to make sure they still behave in a timely manner. Mostly, if you're using an ORM, this is where I suspect you're going to get some pain.
Any questions on specifics, feel free to fire away and I'll elaborate where I can.