edit: my application is "only" for datalogging, so that I get a file that has a self-describing format. Important features for me are being able to add arbitrary metadata, having fast write access for appending to byte arrays, and having single-writer/multiple-reader concurrency (strongly preferred but not a must-have. NetCDF docs say they have SWMR but don't say whether they support any mechanism for ensuring that two writers can't open the same file at once with disastrous results). I like the hierarchical aspect of HDF5 (in particular I love the directed-acyclic-graph hierarchy, much more flexible than a "regular" filesystem-like hierarchy), am reading the NetCDF docs now... if it only allows one dataset per file then it probably won't work for me. :(
update — looks like NetCDF-Java reads from netCDF-4 files but only writes from netCDF-3 files which don't support hierarchical groups. darn.
update 2009-Jul-14: I am starting to get really upset with HDF5 in Java. The library available isn't that great and it has some major stumbling blocks that have to do with Java's abstraction layers (compound data types). A great file format for C but looks like I just lose. >:(