I've been using the previous version of the library (JDBM3) for some time now, although I haven't tried MapDB just yet. They have very similar APIs, however.
I've found JDBM3 a pleasure to use; it's fast and stable, with an excellent API. It's really quite easy to start using (it exposes Java collection interfaces), but you can configure it to do some powerful things under the hood. I'm not using the library in a really high-performance setting (just need a persistent key-value store), so I can't quite comment on the extreme scaling qualities.
I've found JDBM3 a pleasure to use; it's fast and stable, with an excellent API. It's really quite easy to start using (it exposes Java collection interfaces), but you can configure it to do some powerful things under the hood. I'm not using the library in a really high-performance setting (just need a persistent key-value store), so I can't quite comment on the extreme scaling qualities.
I'm excited to switch to MapDB as it matures.