It depends want you want to do. If you want to build a simple more "portal" style interface to Alfresco then they are ideal. As you build more complex screens then there is more work to do - but as long as the APIs are available in JS/FreeMarker (such as People, Workflow, AVM are all there) then it's doable. Yes of course it is time consuming to re-implement functionality, but if you want a different UI then that's how it has to be unless you drop back into the existing web-client.
Thanks,
Kevin