I have successfully passed the two mysql developer exams and i'm now a Sun Certified MySQL Developer.
Currently i'm also preparing myself to become a certified DBA.
During the process of getting certified i ran into some annoyances that are the result of the take-over by Oracle. It started with a missing errata page and after a search (including a lot of emails and a lot of phone calls) of several weeks i finally got an official answer from oracle which made it clear that the errata page of the mysql certification study guide had disappeared forever.
The sections below describe what happened. And because Oracle won't, i decided to revive the errata page myself.
Unfortunately, things have changed since i started with the developer exams. Back then MySQL was part of Sun, but as you all know Sun was bought by Oracle which makes MySQL a part of Oracle now.
Becoming certified was a simple process. MySQL offered all the information you needed to know on their website and dedicated a special section to their study guide - the MySQL Certification Study Guide.
The MySQL 5.0 Certification Study Guide is written by (former?) MySQL employees (people from the documentation and certification team) and it really teaches you all the ins and outs you need to know for the exam. Furthermore it contains a CD with questions that help you prepare.
It was not my first exam and i can tell you: the questions are very challenging. But, I can also assure you: if you read (and memorize) the information in this book, you will pass the exams without doubt. It is really very good and easy to understand.