In my point of view, the RDF-Query provides an important contribution towards efficient processing of SPARQL queries. The package is easy to use in terms of having it to work for RDF::Redland or RDF::Core.
However, although the purpose of RDF-Query is to work on a certain schema, I think it is still important for it to work on other ones as well. With the increase in the number of datastores for RDF data it is more of a pressing issue to have an "extendable" translator for SPARQL to SQL. Unfortunately, personally, I didn't find it easy to extend RDF-Query to work on another schema.
Overall, I think it's a positive step forward towards scalable and efficient processing of SPARQL queries.