> db.bindata.insert(

{ foo: BinData(0,"gpJOnIbX+ZJkAwTZpAw=")}

)
> db.bindata.find()

{ "_id" : ObjectId("4e9d87aa5825b60b637815a6"), "foo" : null }

so BinData(0,"gpJOnIbX+ZJkAwTZpAw=")produces null..

> db.bindata.insert(

{ foo: HexData(0,"5555")}

)
> db.bindata.find()

{ "_id" : ObjectId("4e9d87aa5825b60b637815a6"), "foo" : null } { "_id" : ObjectId("4e9d87d15825b60b637815a7"), "foo" : BinData(0,"VVU=") }

whereas HexData(0, "5555") produces a non-null value, as expected.
If I use the new keyword with BinData, it does what I expect:

> db.bindata.insert(

{ foo: new BinData(0,"gpJOnIbX+ZJkAwTZpAw=")}

)
> db.bindata.find()

{ "_id" : ObjectId("4e9d87aa5825b60b637815a6"), "foo" : null } { "_id" : ObjectId("4e9d87d15825b60b637815a7"), "foo" : BinData(0,"VVU=") } { "_id" : ObjectId("4e9d87e15825b60b637815a8"), "foo" : BinData(0,"gpJOnIbX+ZJkAwTZpAw=") }

and..

> db.bindata.find(

{foo: new BinData(0,"gpJOnIbX+ZJkAwTZpAw=")}

)

{ "_id" : ObjectId("4e9d87e15825b60b637815a8"), "foo" : BinData(0,"gpJOnIbX+ZJkAwTZpAw=") }

so, basically "new" needed with BinData, but not HexData.