It supports connecting to PostgreSQL and MSSQL currently. When you connect to an external engine it opens up a connection, and copies selected Excel tables as temp tables to the destination DB. This makes it easy to combine excel data with existing db data in your queries, and also to move data in easily in either direction.

It uses low level bulk copy operations when importing excel data so it's pretty fast. I did not yet implement support for Oracle because it doesn't have session level temp tables, only global temp tables... I'll see what I can do there.

I haven't used the MySQL Excel plugin so I can't comment on its features...