up-to-date. The mongo driver is pretty well written. The difference can be
seen nere:

    public CommandResult doEval( String code , Object ... args )
        throws MongoException {

        return command( BasicDBObjectBuilder.start()
                        .add( "$eval" , code )
                        .add( "args" , args )
                        .get() );
    }

    public Object eval( String code , Object ... args )
        throws MongoException {

        CommandResult res = doEval( code , args );

        if ( res.ok() ){
            return res.get( "retval" );
        }

        throw new MongoException( "eval failed: " + res );
    }

So eval simply wraps doEval, does a check for successful execution and
returns the "retval" value from the result.

I used eval because I wanted the result of the function call. You could
probably use eval for both calls - but once I got it working I didn't
investigate much more. I don't use eval any more - I replaced it with
findAndModify.

What I haven't found yet is much documentation on all the commands you can
execute with db.command, and their args :(

I have not experienced this, and I fully plan to go into production using
Mongo in the next month or so.  Do you have any further details as to where
it's loosing data? I know about the following...

   - if you don't use WriteConcern.SAFE then it is possible for writes to
   fail and you will not be notified
   - if you write a lot and then query before the writes are all on disk
   then some may not be there yet
   - replication is necessary if durability is a requirement
   - you need a good optimistic-locking strategy if concurrent modifications
   are possible and you can't use the limited set of operation modifiers,
   otherwise later changes can overwrite previous changes

Are you aware of any other potential problems?