It depends on the use and implementation, as I prefer to use Galera or XtraDB because of replication and failover functionality. However, the ability to scale horizontally is much better than other vendors. Also, the transactional functionality is good.
Room for Improvement:
Configuration templates and reliability both need work.
Use of Solution:
I have been using MySQL for over five years.
I deploy multiples of these servers now, and it is all managed via a puppet so I/we have not had problems with deployment in a long time.
An overload of sessions, or long running queries. can cause the servers to be unusable.
Currently no, as we use our own load balancer to enable us to scale to any level.
It was straightforward, and there are numerous blogs on-line that cover every possible configuration.
It takes about six months, but this does depend on its associated product and usage.
Cost and Licensing Advice:
Chose your database based on traffic type and desired functionality, not on the initial cost.
If you have the in-house knowledge MySQL can be a great product. If you do not I would recommend using MSSQL instead.
Take a look at the environment you plan to implement including the application and traffic model.