The main difference in version 0.5.5 from the previous one is the inclusion of a RDF molecule store. Both in memory and disk based versions are supported and can be queried just like a normal triple store. This is also the first version that has been renamed URQL instead of SPARQL for the query evaluation. The SPARQL grammar is the same but it does not support the weird outliers that SPARQL has for empty graph patterns but follows relational (and other) algebras. There's also the usual bug fixes and other features.
Update: Due to a couple of bugs found in 0.5.5 there will be a 0.5.5.1 version released soon.