Some pretty positive reviews! Here are some treasure quotes. Note the range from embedded to client/server, with some very heavy data loads. The amazing thing is that these are some heavy users, and the derby-user list is very quiet, maybe 10 emails a day. It Just Works (TM).
The sign of a good embedded database system is not who’s running it, but rather who doesn’t know they’re running it! The ideal database is one that sits there in the background, working hard, and never raising a peep nor complaining. That’s Apache Derby!
Our software has been deployed by tens of thousands of organizations ranging from small schools all the way up to prestigious academic organizations and corporate banks. Although we offer the option to choose other external databases, the majority stick with Apache Derby as the embedded default. Many PaperCut NG installations handle thousands of print transactions a day and host datasets in the millions of rows. Apache Derby has proven that an embedded zero-administration database can be robust and scalable in a range of environments and operating systems.
I'm currently using derby for a distributed client architecture. I've tested with 3~5 million records that need to be keep in sync through all the clients. Derby has worked beautifully, selects, inserts all very fast. And I've even seen several searches with the same indexes run faster on Derby than Oracle.
We use derby in our search engine project which is in beta stage. Having tried alternatives (mysql , postgre , oralce ) derby provieded us unmatched performence over large datasets our current dataset is around 30GB for each server we read more than a million rows at a time and insert around 300 records per second we are confident that derby will not let us down along the way. we are planning to hit over 100GB mark for both of our servers during the next couple of months