The ones I've spent most time with were Jena/TDB, Virtuoso, 3store, along with a couple of proprietary engines. BigOWLIM is also a strong contender in the space. I've used them in the context of both object storage and semantic web data storage.
My experience is that if you don't need constraints/enforced relational integrity, RDF stores make for really simple/easy object storage. There's definitely a performance tradeoff, though - depends on what you need, really!
I was looking into using Stardog for a metadata repository I was building, but we ended up (probably unwisely) bastardizing Postgres into a bunch of self-join heirarchies.