I use it currently.
It has many advanced features (multiversioning, stored procedures, triggers, views, server side cursors, referential integrity, ...).
It is very stable, I've ported a critical application of my company to it (a lot of inserts/updates/queries - and a lot of table corruption reports) and I can say that is rock solid speaking about corruption. Not a single corruption report in 4 months.
Also has online backup capabilities and database shadowing (an exact copy of a database mantained separately)
Because its compatibility with Interbase, a lot of free and commercial tools are available.

And it is 100% free, even distributed with commercial applications.

>Anyone played with this?
>
>http://www.firebirdsql.org/