Currently, the only attribute types supported are strings, numbers and booleans. Attempts to use other datatypes are met with:
Assertion Failed: Attributes must be numbers, strings or booleans, not <type>
EmberData already has a mechanism to define how objects are serialized. I would expect that views use the serializer that is already defined for the datatype. If the view requires a different serialization format, then by all means, add another computed property. But barring further customizations, a custom datatype (that already knows how to serialize itself) should be supported by Ember views. At the very least, invoking .toString() on the attribute would be a reasonable default.
As it stands, the value of a custom datatype is greatly diminished if I have to define its serialization mechanism twice (in the Transform and a computed property just for the view).