This is a really important issue.  For those of you that have not tried it, the new unit system is amazing.  It can be slightly confusing at first, but the unit conversions work extremely well once the frontend has knowledge of the native unit system of the data.  The problem is really making sure that the native unit system is understood correctly.  There are cosmology comoving/proper frame issues as well as the fact that some systems define a velocity unit that is independent of the length and time units.  I think maybe the best way to do this is to have a very simple dataset for each frontend in which some basic quantities (velocities, densities, etc) are known and use them for unit testing.  I think a good sample dataset would be a uniform density/velocity grid, perhaps one with cosmology on and one off.  I'm sure this could be made for Enzo.  What about the rest of the codes?