MySQL is an excellent RDBMS. I have been working with MySQL for over 10 years. I have used it for small personal projects, and very large deployments (Singlesnet.com). I have deployed large replicated MySQL clusters, and have experience with all operational aspects of the deployment.
I had a team of people work with me on some scalability research. We tested various ways to get more performance from MySQL by focusing on the underlying hardware platform, SSDs, RAM, and Linux kernel tuning. We tested many storage engines, each with the different hardware and raid configurations that we defined. We published and spoke about our findings at the 2008 MySQL conference.