If I'm not mistaken, the story goes like this: (I've used MySQL, but not Hypersonic)
MySQL is a free 'real' database in the sense that it is used in many production systems, even though it is missing a few common database systems.
Hypersonic is a memory-resident
test database. The fact that all data is kept in memory accounts for the speed difference. I doubt this database would behave well in real situations and personally I wouldn't trust it. Feel free to use it in testing since it is handy, don't use it in production.
Dave