I am new to Janusgraph. By reading the Docs I understand that Janusgraph needs KV or KVC as the backend. But if I want to use ES as backend, what kind of work is needed? Does it really make sense to do so? Any suggestions/comments are appreciated!

There was a recent thread where a developer was exploring using Solr as the storage backend for JanusGraph.

I'd suggest looking over the code for the in-memory or BerkeleyJE storage backends to get an idea of how to implement it. You could also check out Ted's recent work on exploring FoundationDB as the storage adapter.

Keep us updated on your progress if you decide to pursue it!