We just migrated from Sphinx to using the full text search indexes in PostgreSQL, we had to deal with some changes in how special characters are handled, but it's worked well enough.
As far as I remember few years ago they didn't have BM25 and even TF-IDF support. Have they added that? Are you experiencing any issues with full-text search quality after migrating from Sphinx (you probably used BM15(+F) which is BM25 w/o doc length).
Do you have any numbers on your requests or searches per second in a real use case? I've really been wondering this as I've been considering manticore which is the major sphinx fork.