All our users I know of to use Derby are small companies. My rough estimate is that they may have several tens of thousands of records in a single table and a few concurrent users (probably less than 5). We did not get reports of any database or multi-user related issues.
In our own tests we successfully batch-created several hundreds of thousands of instances (under a million, I think) using different databases, and measured their performance. MS SQL Server was the quickest, then MySQL (about twice as fast as Derby). However, this difference in performance may not necessarily be as obvious, or even noticeable, to the end user performing normal data search/create/edit operations on a client machine.
In our latest test of Aware IM with Derby we used a specially configured Aware IM application with a process that batch-creates 10,000 instances to gradually increase the number of records in a table from 0 to 100,000. After each process run we used the browser to manually create a new instance and perform a text search using specific criteria. The results were as follows. The creation of a single new instance was as quick (almost instantaneous) on a 100,000 records table as on an empty table. There was no noticeable slowing down in search up to about 50,000 records, which took under 1 second. With 100,000 instances the search took about a second longer with the overall time still under 2 seconds. Note that these results include the browser page refresh, browser-server communication and Aware IM Server processing time, all of which take place outside of the database. The computer used in this test had AMD Athlon CPU at 1.6 GHz and 512MB of RAM. Both the Aware IM Server and the browser (the operation client) were running on the same machine.
Let me know if you would like to reproduce this test on your own hardware to get a feeling of what the performance is really like.
Also, please keep in mind that if for whatever reasons an Aware IM application needs to be moved to a different Aware IM-supported database (more powerful, different location, etc.) it will be possible to do it at a later date.
Let me know if this information helps you in your evaluation exercise.