Hadn't considered Riak, but I can see that it has some full-text search capabilities. Any idea about its features and how it compares in performance, as a raw search index?
If it's just Solr underneath, then why is the pesudo-Solr API implementation not a complete implementation? Something to do with each node being an isolated Solr instance maybe?
There is backward compatibility with the old Riak Search (which wasn't Solr based) intended to not break old applications, but you can query with any currently implemented Solr client afaik.
I don't know of any publicly available relative raw performance benchmarks, and haven't done any myself. My guess is that the compelling features would be more in the realm of node operations and recovery from node failures.
Edit: Apparently my Riak knowledge is dated now anyway. It looks like I have some research to do myself, but it's pretty exciting stuff.
[1]: http://www.christopherbiscardi.com/2014/02/07/geospatial-ind...